Perform the suggestions mentioned in the following articles:

• Make sure that you are using the "Remember History" setting as described in this article -> Settings for privacy, browsing history and do-not-track
• Clear Cookies & Cache
• Clear the Network Cache

-> Start Firefox in -> Diagnose Firefox issues using Troubleshoot Mode to check if your add-ons/extensions are causing the problems.

• Also see this article -> Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its working normally while using Firefox in Diagnose Firefox issues using Troubleshoot Mode.

Which email site are you having problems adding attachments to emails? Yahoo mail, Hotmail, Gmail, AOL webmail or something else?

The site is the webmail client for my own domain name. It uses Roundcube but I have a couple of other options such as Squirrel Mail. However, again I'm not having the problem with IE, so I think it's a Firefox issue (or perhaps the way Firefox interacts with my webmail client), not a problem inherent in my webmail client. And it's occurred only since the update to Firefox 12.

Have you tried to perform the suggestions mentioned in my first reply ??

Sorry I didn't say. Yes, I tried all of them. No luck.

Try Resetting actions for File Types as described in these articles:

• Reset download actions for all file types
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

Also see this article -> Applications panel - Set how Firefox handles different types of files

Check and tell if its working.

See also:

• http://www.winhelponline.com/articles/195/1/How-to-clear-the-file-names-MRU-in-Common-Dialog-boxes-in-Windows-Vista.html

Try to delete the MRU (Most Recently Used) ComDlg32 registry keys with the regedit.exe program.
Navigate to: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32
Delete the subkeys (LastVisitedPidlMRU and OpenSavePidlMRU)

I'm sorry; I misspoke. I had tried mha007's solutions except troubleshooting my Add-ins. I realized I had one add-in that was disabled (probably hadn't been updated for current version of Firefox), and when I uninstalled it, the attachment problem seemed to be corrected. At least for now...I'll be back if it recurs! I really do appreciate everyone's suggestions.