Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

While still in Safe Mode,

Use a safe link. Is the problem still there?

Thanks Fred. The problem persists in safe mode.

Also, if no Firefox windows are open, dragging a URL to the FF dock icon, I get 3 duplicate new windows rather than 3 duplicate tabs.

I've found that a small proportion of URLs will only open 2 dupes. (Most open 3). 2 vs 3 behaviour is consistently repeatable on a per URL basis.

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.

rorynz said

I've found that a small proportion of URLs will only open 2 dupes. (Most open 3). 2 vs 3 behaviour is consistently repeatable on a per URL basis.

Open the properties of the links you are using. Do they look okay?

The links are fine. I've yet to find a single link that doesn't open duplicates. Even links in the emails from this Mozilla support thread do it.

FredMcD said

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there? Starting The Computer In Safe Mode;
Free Online Encyclopedia

---

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.

Safe Mode didn't make any difference. (Note the problem persisted through a clean OS install and fresh download & install of the browser, which surprised me).

Application Basics

Name: Firefox Version: 42.0 Build ID: 20151029151421 Update Channel: release User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:42.0) Gecko/20100101 Firefox/42.0 Multiprocess Windows: 0/1 (default: false) Safe Mode: false

Crash Reports for the Last 3 Days

All Crash Reports

Extensions

Name: Adblock Plus Version: 2.6.11 Enabled: true ID: {d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}

Name: Cookie Controller Version: 4.4 Enabled: true ID: {ac2cfa60-bc96-11e0-962b-0800200c9a66}

Name: NoScript Version: 2.6.9.39 Enabled: true ID: {73a6fe31-595d-460b-a920-fcc0f8843232}

Name: Search by Image for Google Version: 1.2.0.1-signed Enabled: true ID: {ab4b5718-3998-4a2c-91ae-18a7c2db513e}

Name: BetterPrivacy Version: 1.68.1-signed Enabled: false ID: {d40f5e7b-d2cf-4856-b441-cc613eeffbe3}

Graphics

Asynchronous Pan/Zoom: none Device ID: 0x0fd5 GPU Accelerated Windows: 1/1 OpenGL (OMTC) Supports Hardware H264 Decoding: false Vendor ID: 0x10de WebGL Renderer: NVIDIA Corporation -- NVIDIA GeForce GT 650M OpenGL Engine windowLayerManagerRemote: true AzureCanvasBackend: skia AzureContentBackend: quartz AzureFallbackCanvasBackend: none AzureSkiaAccelerated: 0

Important Modified Preferences

browser.cache.disk.capacity: 358400 browser.cache.disk.filesystem_reported: 1 browser.cache.disk.smart_size.first_run: false browser.cache.frecency_experiment: 3 browser.download.importedFromSqlite: true browser.places.smartBookmarksVersion: 7 browser.sessionstore.upgradeBackup.latestBuildID: 20151029151421 browser.startup.homepage: file:///Users/rory/Sites/insecureBookmarks.html browser.startup.homepage_override.buildID: 20151029151421 browser.startup.homepage_override.mstone: 42.0 dom.apps.reset-permissions: true dom.mozApps.used: true extensions.lastAppVersion: 42.0 media.gmp-gmpopenh264.abi: x86_64-gcc3-u-i386-x86_64 media.gmp-gmpopenh264.lastUpdate: 1447958617 media.gmp-gmpopenh264.version: 1.5.1 media.gmp-manager.buildID: 20151029151421 media.gmp-manager.lastCheck: 1448052972 network.cookie.prefsMigrated: true network.predictor.cleaned-up: true places.database.lastMaintenance: 1447960627 places.history.expiration.transient_current_max_pages: 104858 plugin.disable_full_page_plugin_for_types: application/pdf plugin.importedState: true privacy.sanitize.migrateFx3Prefs: true storage.vacuum.last.index: 0 storage.vacuum.last.places.sqlite: 1447960627

Important Locked Preferences

JavaScript

Incremental GC: true

Accessibility

Activated: false Prevent Accessibility: 0

Library Versions

NSPR Expected minimum version: 4.10.10 Version in use: 4.10.10

NSS Expected minimum version: 3.19.4 Basic ECC Version in use: 3.19.4 Basic ECC

NSSSMIME Expected minimum version: 3.19.4 Basic ECC Version in use: 3.19.4 Basic ECC

NSSSSL Expected minimum version: 3.19.4 Basic ECC Version in use: 3.19.4 Basic ECC

NSSUTIL Expected minimum version: 3.19.4 Version in use: 3.19.4

Experimental Features

I've called the big guys to help you. Good luck.

Important update: I was seeing this issue in Firefox and not Safari, so had assumed it was a Firefox issue.

I've now installed and tested with Chrome & Opera and they exhibit the same problem. In other words, only Safari is immune. Therefore, I now think it's a Mac OS X El Capitan Bug, not a Firefox specific bug. The timing of the issue appearing also seems to fit - I started noticing the issue a few weeks ago, around the time that I upgraded to El Capitan from Yosemite.

Any further help would of course be appreciated, but we certainly can't blame Firefox for causing the issue. It's also not specific to the source of the URL. I can drag URLs from Firefox, Apple Mail, Safari etc into the dock icons and get the same issue, even the "Mozilla Support" button at the top left of this page.

Did you try to drag the current Firefox icon to the trash and create a new icon on the dock?

cor-el said

Did you try to drag the current Firefox icon to the trash and create a new icon on the dock?

This makes no difference.

Is Firefox the default browser?

Does this also happen when Firefox isn't the default browser or when you select Firefox as the application to open the file when clicking in the Finder?

Try to uninstall and reinstall Firefox.

Download a new copy of the Firefox application and save the disk image file to the desktop

• Firefox 42.0: https://www.mozilla.org/en-US/firefox/all/

• Trash the current Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean (re)install
• Install the new version that you have downloaded
• https://support.mozilla.org/kb/Installing+Firefox+on+Mac

Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data when you uninstall and (re)install or update Firefox, but make sure NOT to remove personal data when you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data.

Firefox is not the default browser.

Dragging HTML files from Finder (or using Open With) does not exhibit the problem. Only dragging URLs from pages e.g. if I drag the Firefox download blue text link that you posted, from the browser page to the Firefox, Chrome or Opera dock icons, 3 duplicates of it are opened in whichever browser. If I drag to the Safari icon, only one opens.

I think that this would still indicate a problem with the launch icon in the dock.

Are there any specific settings on Mac for what command is used if you drag items to this icon?

Create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I don't know of any such settings. The only system preferences available regarding dock are visual related e.g. size, location, autohide, magnification etc.

I am seeing the issue on a brand new profile. I erased the hard drive and did a completely fresh install of Mac OS and downloaded & installed Firefox onto the virgin Mac OS. I did not restore my old Firefox profile or Mac OS account. For all intents and purposes, I am now using a brand new Mac with just my Documents folder copied over manually from a backup.

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)

Try to repair the disk permissions:

• http://thexlab.com/faqs/repairprocess.html

No change after removing Firefox plist file. Repairing disk permissions isn't a thing anymore on Mac OS. I've run a disk verify and it found no errors.