Installing flash should not effect Firefox except for the plugin.

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

If you were prompted to (re)approve your Kaspersky add-ons, it's possible that Kaspersky updated. But it's also possible that some settings files were "locked" and Firefox therefore behaved like a new install with respect to some things. I don't know whether the Flash installer could have locked those files; usually I don't think it does anything in Firefox's settings folder.

I'm glad to hear it cleared up after closing and restarting Firefox because rebuilding from scratch definitely is no fun. I think Fred's advice still makes sense because some other process may have touched your settings.

More here: https://support.mozilla.org/en-US/questions/1085300

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Kaspesky now combing C drive. I also get a message that posting this on this page is forbidden.

Posting on what page? And by what?

FredMcD said

Posting on what page? And by what?

This page. I had to create a new account to get back in here, since my passwords are definitely lost (https://support.mozilla.org/en-US/questions/1085300 ). When I tried to post the answer above, I got a "You are forbidden to access [or something like that] this page". I tried again later and it worked. Probably was due to the create new account procedure - I was logged in over there (https://support.mozilla.org/en-US/questions/1085300 ) but maybe not here.