hello peterjay321, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

The sound disappears in safe mode but it has nothing to do with the Add-ons as it is there when they are all disabled. My antivirus and malware protection are all up to date and have detected nothing.

could you go to the firefox menu ≡ > help ? > troubleshooting information, copy the contents of that page and paste them here into a reply on the forum? this might give us a clue what is going on...

Troubleshooting Information

Application Basics Name Firefox Version 33.1 Update History User Agent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:33.0) Gecko/20100101 Firefox/33.0 Profile Folder Enabled Plugins about:plugins Build Configuration about:buildconfig Memory Use about:memory Multiprocess Windows 0/1 Crash Reports for the Last 3 Days Report ID Submitted

All Crash Reports Extensions Name Version Enabled ID Adblock Plus 2.6.6 true {d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d} Autofill 3.6 true firefox-autofill@googlegroups.com Blur (Formerly DoNotTrackMe) 4.5.1334 true donottrackplus@abine.com eBay Sidebar for Firefox 3.1.5 true {62760FD6-B943-48C9-AB09-F99C6FE96088} Yahoo! Mail Notifier 1.0.7.4 true {89f8dde0-010a-11da-8cd6-0800200c9a66} Hotspot Shield Extension 3.19 false afext@anchorfree.com Graphics Adapter Description Intel(R) HD Graphics Adapter Drivers igdumd64 igd10umd64 igd10umd64 igdumd32 igd10umd32 igd10umd32 Adapter RAM Unknown Device ID 0x0102 Direct2D Enabled true DirectWrite Enabled true (6.2.9200.16571) Driver Date 3-20-2014 Driver Version 9.17.10.3517 GPU #2 Active false GPU Accelerated Windows 1/1 Direct3D 11 (OMTC) Vendor ID 0x8086 WebGL Renderer Google Inc. -- ANGLE (Intel(R) HD Graphics Direct3D9Ex vs_3_0 ps_3_0) windowLayerManagerRemote true AzureCanvasBackend direct2d AzureContentBackend direct2d AzureFallbackCanvasBackend cairo AzureSkiaAccelerated 0 Important Modified Preferences Name Value accessibility.typeaheadfind true accessibility.typeaheadfind.flashBar 0 browser.cache.disk.capacity 358400 browser.cache.disk.smart_size_cached_value 358400 browser.cache.disk.smart_size.first_run false browser.cache.disk.smart_size.use_old_max false browser.cache.frecency_experiment 4 browser.cache.memory.capacity 65536 browser.cache.use_new_backend 1 browser.places.smartBookmarksVersion 7 browser.search.useDBForOrder true browser.sessionstore.enabled true browser.sessionstore.upgradeBackup.latestBuildID 20141106120505 browser.startup.homepage https://uk-mg42.mail.yahoo.com/neo/launch?retry_ssl=1#8125716199|http://www.ebay.co.uk/myb/WatchList?_trksid=p2060778.m2 browser.startup.homepage_override.buildID 20141106120505 browser.startup.homepage_override.mstone 33.1 browser.tabs.drawInTitlebar false dom.mozApps.used true extensions.lastAppVersion 33.1 font.internaluseonly.changed false font.name.serif.x-western Comic Sans MS general.autoScroll false gfx.direct3d.last_used_feature_level_idx 0 keyword.URL http://uk.search.yahoo.com/search?fr=greentree_ff1&ei=utf-8&ilc=12&type=937811&p= media.gmp-gmpopenh264.lastUpdate 1413444549 media.gmp-gmpopenh264.version 1.1 media.gmp-manager.lastCheck 1417972302 network.cookie.prefsMigrated true network.http.max-connections 48 network.http.max-connections-per-server 16 network.http.max-persistent-connections-per-proxy 16 network.http.max-persistent-connections-per-server 8 network.http.pipelining.maxrequests 8 network.http.request.max-start-delay 0 places.database.lastMaintenance 1418031242 places.history.enabled false places.history.expiration.transient_current_max_pages 104858 plugin.disable_full_page_plugin_for_types application/pdf plugin.expose_full_path true plugin.importedState true plugin.state.npaclmplugin 0 plugin.state.npctrl 0 plugin.state.npdeployjava 0 plugin.state.npidfplugin 0 plugin.state.nppitplugin 0 plugin.state.npproductdetectplugin 0 plugin.state.npsitesafety 0 plugin.state.npvlc 0 plugin.state.npystate 0 print.printer_hp_deskjet_5550_series_(HPA).print_bgcolor false print.printer_hp_deskjet_5550_series_(HPA).print_bgimages false print.printer_hp_deskjet_5550_series_(HPA).print_colorspace print.printer_hp_deskjet_5550_series_(HPA).print_command print.printer_hp_deskjet_5550_series_(HPA).print_downloadfonts false print.printer_hp_deskjet_5550_series_(HPA).print_duplex 3211314 print.printer_hp_deskjet_5550_series_(HPA).print_edge_bottom 0 print.printer_hp_deskjet_5550_series_(HPA).print_edge_left 0 print.printer_hp_deskjet_5550_series_(HPA).print_edge_right 0 print.printer_hp_deskjet_5550_series_(HPA).print_edge_top 0 print.printer_hp_deskjet_5550_series_(HPA).print_evenpages true print.printer_hp_deskjet_5550_series_(HPA).print_footercenter print.printer_hp_deskjet_5550_series_(HPA).print_footerleft &PT print.printer_hp_deskjet_5550_series_(HPA).print_footerright &D print.printer_hp_deskjet_5550_series_(HPA).print_headercenter print.printer_hp_deskjet_5550_series_(HPA).print_headerleft &T print.printer_hp_deskjet_5550_series_(HPA).print_headerright &U print.printer_hp_deskjet_5550_series_(HPA).print_in_color true print.printer_hp_deskjet_5550_series_(HPA).print_margin_bottom 0.5 print.printer_hp_deskjet_5550_series_(HPA).print_margin_left 0.5 print.printer_hp_deskjet_5550_series_(HPA).print_margin_right 0.5 print.printer_hp_deskjet_5550_series_(HPA).print_margin_top 0.5 print.printer_hp_deskjet_5550_series_(HPA).print_oddpages true print.printer_hp_deskjet_5550_series_(HPA).print_orientation 1 print.printer_hp_deskjet_5550_series_(HPA).print_page_delay 50 print.printer_hp_deskjet_5550_series_(HPA).print_paper_data 9 print.printer_hp_deskjet_5550_series_(HPA).print_paper_height 297.00 print.printer_hp_deskjet_5550_series_(HPA).print_paper_name print.printer_hp_deskjet_5550_series_(HPA).print_paper_size_type 0 print.printer_hp_deskjet_5550_series_(HPA).print_paper_size_unit 1 print.printer_hp_deskjet_5550_series_(HPA).print_paper_width 210.00 print.printer_hp_deskjet_5550_series_(HPA).print_plex_name print.printer_hp_deskjet_5550_series_(HPA).print_resolution 3473458 print.printer_hp_deskjet_5550_series_(HPA).print_resolution_name print.printer_hp_deskjet_5550_series_(HPA).print_reversed false print.printer_hp_deskjet_5550_series_(HPA).print_scaling 1.00 print.printer_hp_deskjet_5550_series_(HPA).print_shrink_to_fit true print.printer_hp_deskjet_5550_series_(HPA).print_to_file false print.printer_hp_deskjet_5550_series_(HPA).print_unwriteable_margin_bottom 0 print.printer_hp_deskjet_5550_series_(HPA).print_unwriteable_margin_left 0 print.printer_hp_deskjet_5550_series_(HPA).print_unwriteable_margin_right 0 print.printer_hp_deskjet_5550_series_(HPA).print_unwriteable_margin_top 0 print.printer_Samsung_Universal_Print_Driver_2.print_bgcolor false print.printer_Samsung_Universal_Print_Driver_2.print_bgimages false print.printer_Samsung_Universal_Print_Driver_2.print_colorspace print.printer_Samsung_Universal_Print_Driver_2.print_command print.printer_Samsung_Universal_Print_Driver_2.print_downloadfonts false print.printer_Samsung_Universal_Print_Driver_2.print_duplex 3211314 print.printer_Samsung_Universal_Print_Driver_2.print_edge_bottom 0 print.printer_Samsung_Universal_Print_Driver_2.print_edge_left 0 print.printer_Samsung_Universal_Print_Driver_2.print_edge_right 0 print.printer_Samsung_Universal_Print_Driver_2.print_edge_top 0 print.printer_Samsung_Universal_Print_Driver_2.print_evenpages true print.printer_Samsung_Universal_Print_Driver_2.print_footercenter print.printer_Samsung_Universal_Print_Driver_2.print_footerleft &PT print.printer_Samsung_Universal_Print_Driver_2.print_footerright &D print.printer_Samsung_Universal_Print_Driver_2.print_headercenter print.printer_Samsung_Universal_Print_Driver_2.print_headerleft &T print.printer_Samsung_Universal_Print_Driver_2.print_headerright &U print.printer_Samsung_Universal_Print_Driver_2.print_in_color true print.printer_Samsung_Universal_Print_Driver_2.print_margin_bottom 0.5 print.printer_Samsung_Universal_Print_Driver_2.print_margin_left 0.5 print.printer_Samsung_Universal_Print_Driver_2.print_margin_right 0.5 print.printer_Samsung_Universal_Print_Driver_2.print_margin_top 0.5 print.printer_Samsung_Universal_Print_Driver_2.print_oddpages true print.printer_Samsung_Universal_Print_Driver_2.print_orientation 1 print.printer_Samsung_Universal_Print_Driver_2.print_page_delay 50 print.printer_Samsung_Universal_Print_Driver_2.print_paper_data 9 print.printer_Samsung_Universal_Print_Driver_2.print_paper_height 297.00 print.printer_Samsung_Universal_Print_Driver_2.print_paper_name print.printer_Samsung_Universal_Print_Driver_2.print_paper_size_type 0 print.printer_Samsung_Universal_Print_Driver_2.print_paper_size_unit 1 print.printer_Samsung_Universal_Print_Driver_2.print_paper_width 210.00 print.printer_Samsung_Universal_Print_Driver_2.print_plex_name print.printer_Samsung_Universal_Print_Driver_2.print_resolution 3473458 print.printer_Samsung_Universal_Print_Driver_2.print_resolution_name print.printer_Samsung_Universal_Print_Driver_2.print_reversed false print.printer_Samsung_Universal_Print_Driver_2.print_scaling 1.00 print.printer_Samsung_Universal_Print_Driver_2.print_shrink_to_fit true print.printer_Samsung_Universal_Print_Driver_2.print_to_file false print.printer_Samsung_Universal_Print_Driver_2.print_unwriteable_margin_bottom 0 print.printer_Samsung_Universal_Print_Driver_2.print_unwriteable_margin_left 0 print.printer_Samsung_Universal_Print_Driver_2.print_unwriteable_margin_right 0 print.printer_Samsung_Universal_Print_Driver_2.print_unwriteable_margin_top 0 privacy.donottrackheader.enabled true privacy.popups.showBrowserMessage false privacy.sanitize.migrateFx3Prefs true privacy.sanitize.timeSpan 0 storage.vacuum.last.index 1 storage.vacuum.last.places.sqlite 1415446242 Important Locked Preferences Name Value JavaScript Incremental GC true Accessibility Activated true Prevent Accessibility 0 Library Versions Expected minimum version Version in use NSPR 4.10.7 4.10.7 NSS 3.17.2 Basic ECC 3.17.2 Basic ECC NSSSMIME 3.17.2 Basic ECC 3.17.2 Basic ECC NSSSSL 3.17.2 Basic ECC 3.17.2 Basic ECC NSSUTIL 3.17.2 3.17.2 Experimental Features Name ID Description Active End Date Homepage

the Yahoo! Mail Notifier extension comes with the option to play an audio alert once you receive new mail...

Yes I know about that, which makes a totally different sound, similar to a door bell chime.