Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

Please report back soon.

Is Open or Open Containing Folder enabled or is it dimmed (disabled)?

If the latter is the case and those entries are grayed then the files aren't in that download location and might have been blocked (removed) by security software.

ComputerWhiz, thanks for the suggestion, but the problem persists in Safe Mode.

cor-el, I can still use the "Open Containing Folder" (it is enabled), but I don't see an "Open" option, neither in the panel or library. The only options that are available are "Remove From History", "Open Containing Folder", "Go To Download Page", "Copy Download Link" and "Clear List"

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
• TDSSKiller - AntiRootkit Utility
• 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 soon.

I scanned the computer with the Microsoft Safety Scanner, and no threats were found.

I hadn't tried it yet, but it turns out that using a different profile (clean), solves the problem. Tomorrow I'll try backing up and restoring my profile in order to clean it, or something like that. If you have any ideas to solve the problem in a simple way, I'll check and accept the answer if they work.

Thanks

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions and some Preferences.

• Open websites will not be saved in Firefox versions lower than 25.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back soon.

Thanks for the suggestions.

I considered resetting my firefox (it would probably have solved it), but I was able to track the problem down to mimetypes.rdf. I deleted it and now everything is fine.

Solution: Deleting mimetypes.rdf OR Resetting (see this answer)

I am glad to hear that your problem has been resolved.

I hope you continue using our products and thank you for contacting Mozilla Support.