Firefox should not do that unless it was told to.

Type about:preferences in the address bar and press enter

Select Applications and see if those file types are listed.

Yes, RAR and MKV are listed. As I mentioned, I've used Firefox to download such file types.

I also have the "Open with: Firefox" option for other file types that aren't listed, like TXT. I suppose this could be intentional, but this option is of dubious use since Firefox can only display text files, not edit them.

Yet I don't have the "Open with: Firefox" option for other file types that are listed under Options → Applications, like TORRENT.

Thank you for the reply.

Rename (or delete) the mimeTypes.rdf file in the Firefox profile folder to reset all file actions.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Deleting mimeTypes.rdf had no effect apart from resetting the list of file types under Options → Applications. I've restarted Windows to be sure. I suppose it's to be expected, since Windows Explorer context menu information is stored in the Windows registry.

I uninstalled Firefox so I could delete Firefox-related things from the registry without breaking something. I came across firefox.exe listed in keys like the following. Yes, that's MSI in the list: Firefox was listed as an option for opening executable files.

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.7z\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.ini\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.mkv\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.msi\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.psd\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.wav\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.wmv\OpenWithList
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.zip\OpenWithList

As mentioned in the Mozillazine thread, using the NirSoft utility OpenWithView is quicker — and safer, if you're not experienced with registry editing.

• http://www.nirsoft.net/utils/open_with_view.html

GbM,

There's something screwed up in Firefox 31 and 32 with regards to Firefox messing with Windows OS File Associations in WinXP and Win7. Seems to have been reported by a lot more WinXP users, but a few Win7 users have reported the same.

It makes sense that it is happening at the Registry level rather than being within the Firefox Profile files.

Being Sunday, I'm too lazy to search for the relevant Bug report.

Me thinks me smells a virus at work.

Possible Mal-Ware Scan For Macs {web link}

For Windows Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

After cleaning the registry manually, I haven't been able to trigger this.

• I now have RAR under Options → Applications again after downloading a file of that type. The respective file type doesn't have Firefox under the "Open with" menu in Windows Explorer.
• I set Firefox as the default browser when installing. Then I set Internet Explorer as the default using the Control Panel, then Firefox again when prompted at startup. Since it's one of the few times Firefox interacts with the registry, I thought it might trigger the issue — it didn't.

Since I can't recreate this, I think we can consider it a rare occurrence and forget about it. Thank you all for replying.

the-edmeister wrote:

There's something screwed up in Firefox 31 and 32 with regards to Firefox messing with Windows OS File Associations […]

Being Sunday, I'm too lazy to search for the relevant Bug report.

• Bug 1049521 - Be less aggressive about non-primary file type associations in Firefox on Windows

That's the bug report about Firefox inadvertently stealing the file association for PDFs. That didn't happen on my system. I also don't see how this could be related. The PDF association is set on install, and reinstalling Firefox didn't change the "Open with" list for the previously affected file types. Then again, the patch on bug 1049521 touches the OpenWithList registry keys, which is what I manually cleaned up. It also handles more than PDFs: I spotted media file types like OG? and WEBM, but not MKV or any of the previously affected file types on my system.

FredMcD wrote:

Me thinks me smells a virus at work.

The system is clean.

Viruses are know to change file associations. That is why I said that.

Once you are done with fixing the registry, you should whip all backups, and create a new one.

I've filed a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1077429