System:

• Windows 7 64-bit Enterprise Edition
• OS Version 6.1.7600 Build 7600
• Dell Optiplex 980

Functionality in Firefox 18.0.1:

• Google Search works
• Google Images work with images displayed
• External links invoked from within Google Search Results fail
• Internet Explorer and Chrome work fine

Troubleshooting:

• Uninstalled all addins
• Disabled Windows Firewall
• Disabled Antivirus
• Disabled Windows DEP
• Disabled Windows UAC
• Rebooted PC and closed all non-essential processes
• Increased network connection timeout in Firefox in about:config
• Tested Auto-detect proxy settings in Firefox (Advanced >> Network >> Settings)
• Tested Use system proxy settings (Advanced >> Network >> Settings)
• Tested manually-entered proxy details (Advanced >> Network >> Settings)
• Currently using 'Automatically detect' and 'Use automatic config script' in Control Panel >> Internet Options >> Connections >> LAN Settings
• Restored Default Firefox Settings
• Uninstalled Firefox, deleted personal data, deleted installation directory, deleted registry keys and reisntalled 18.0.1 from a newly downloaded Setup Executable
• Tested the above network connection changes with the reinstallation

After all that, I uninstalled Firefox, deleted personal data and installed Firefox 17.0esr. Using the default 'use system proxy settings' network setting, it works perfectly. After upgrading to 17.0.2, it still worked.

When entering an IP address (to test whether it's a DNS resolution issue), 17.0.2 would work and 18.0.1 would still fail, leading me to conclude that the issue is a software bug rather than a proxy problem.

Have the Developers any input as to what may be different between 17 and 18 with regard to basic conectivity?

NB: I should note that I have already trawled through the generic connectivity troubleshooting steps Mozilla provide, as well as the help articles for situations in which Google is the only accessible site, and various other troubleshooting suggestions I found when searching. All signs point to the issue being a bug in this version.

Here is the Troubleshooting Information from 18.0.1:

 Application Basics

       Name
       Firefox

       Version
       18.0.1

       User Agent
       Mozilla/5.0 (Windows NT 6.1; WOW64; rv:18.0) Gecko/20100101 Firefox/18.0

       Build Configuration

         about:buildconfig

 Extensions

       Name

       Version

       Enabled

       ID

 Important Modified Preferences

     Name

     Value

       browser.cache.disk.capacity
       358400

       browser.cache.disk.smart_size.first_run
       false

       browser.cache.disk.smart_size.use_old_max
       false

       browser.cache.disk.smart_size_cached_value
       358400

       browser.places.smartBookmarksVersion
       4

       browser.startup.homepage_override.buildID
       20130116073211

       browser.startup.homepage_override.mstone
       18.0.1

       extensions.lastAppVersion
       18.0.1

       network.cookie.prefsMigrated
       true

       places.database.lastMaintenance
       1359418610

       places.history.expiration.transient_current_max_pages
       104858

       privacy.sanitize.migrateFx3Prefs
       true

 Graphics

       Adapter Description
       NVIDIA GeForce 9300 GE

       Adapter Drivers
       nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um

       Adapter RAM
       256

       Device ID
       0x06e0

       Direct2D Enabled
       true

       DirectWrite Enabled
       true (6.1.7600.16972)

       Driver Date
       10-2-2012

       Driver Version
       9.18.13.697

       GPU #2 Active
       false

       GPU Accelerated Windows
       1/1 Direct3D 10

       Vendor ID
       0x10de

       WebGL Renderer
       Google Inc. -- ANGLE (NVIDIA GeForce 9300 GE)

       AzureCanvasBackend
       direct2d

       AzureContentBackend
       direct2d

       AzureFallbackCanvasBackend
       cairo

 JavaScript

       Incremental GC
       true

 Accessibility

       Activated
       false

       Prevent Accessibility
       0

 Library Versions

       Expected minimum version

       Version in use

       NSPR
       4.9.4
       4.9.4

       NSS
       3.14.1.0 Basic ECC
       3.14.1.0 Basic ECC

       NSSSMIME
       3.14.1.0 Basic ECC
       3.14.1.0 Basic ECC

       NSSSSL
       3.14.1.0 Basic ECC
       3.14.1.0 Basic ECC

       NSSUTIL
       3.14.1.0
       3.14.1.0

See:

• http://kb.mozillazine.org/Error_loading_websites

Unfortunately I've effectively covered the steps there.

• Thoroughly tested Proxy settings
• Disabled Firewall
• Disabled Security apps
• IPv6 is enabled and works fine in FF17
• There's no DNS prefetching option
• I've entered an IP address negating DNS resolution faults
• There are no router issues from the workstation
• Etc.

I skimmed through the remaining sugesstions, but they all relate to infrastructure faults, of which there are none. The issue is very likely caused by a software change, as the immediate previous iteration of the product worked.

Seems this question is dead. Shame one of the Mozilla Devs couldn't give feedback :(