AVG recently updated both its paid and free versions to make the "Safe Search" add-on compatible with Firefox 5.

FREE Release Notes:
http://forums.avg.com/ww-en/avg-free-forum?sec=thread&act=show&id=110701 - "10.0.1388"

PAID Release Notes:
http://www.avg.com/ww-en/avg-release-notes - "2011.1388 Hotfix 2011-07 "

Does that repair the "Online Shield" as well?

Thanks for your response. I should have mentioned in my original post that 1) my version of AVG is paid, and 2) the very first thing I did this morning when I decided to go after the problem was to check for to an update on AVG, figuring they'd heard of this and fixed it. However, my AVG is up to date (it updates every morning automatically) and the problem has not resolved. I have also requested help directly from AVG, just before searching and posting here; I'm thinking I'll hear something Monday. The original error message I got when updating Firefox also pertained to Safe Search, but I never had a problem with it once the update was completed as I did with the Online Shield.

My AVG safe search add-on is still disabled, everything from AVG is updated.

????????

Firefox may not have detected that it was updated. Can you visit this dialog and try to Enable it manually?

orange Firefox button or Tools menu > Add-ons > Extensions category

It may be on the lower part of the list in grayed text below the currently active extensions.

Does that work?

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Also, if you haven't restarted Firefox since you updated AVG, that might be necessary. Firefox usually won't refresh extensions without a restart.