Found following articles that AVG also not compatible with FF4 update: http://answers.yahoo.com/question/index?qid=20080512192747AAHDZtL

I was also having an issue with outlook and syncing my iPhone with iTunes new update so total mess. Above link did stop General Failure message but then did system restore and problem is back. I'm going to try and revert to 3.6 cuz probem definitely started with FF4 update

This is a known bug, and has been for years. It occurs when Firefox is your default browser and some glitch (or program) changes that default. Thankfully, there's a fairly easy fix, though it does involve monkeying around in the Registry.

(Note: These instructions apply to Windows Vista and 7 only.)

1. Click Start, type regedit, and then press Enter.

2. Navigate to the following entry: HKEY_CLASSES_ROOT\FirefoxURL\shell\open\ddeexec

3. In the center pane, double-click the Default entry, then remove whatever value is there (it'll probably be a weird string of numbers and commas).

4. Click OK, then exit Regedit.

5. If Outlook and/or Firefox are open, close them, then restart them. this information was found at[[ http://www.pcworld.com/article/200103/fix_outlook_general_failure_error_for_email_links.html]] I am using windows 7 pro this issue has drove me crazy for some time, now finally it's fixed by doing the exact same steps as above, hope this works for everybody that is having this problem. I just had to share this with my fellow firefox users. :)

Thanks...this worked for me. Upgrading to Firefox 6.0.2 somehow caused this issue, but it it OK now.