Yahoo is gradually taking over my system

I recently updated Flash etc. when asked, but i also installed a Firefox add-on called YouTube Unblocker, which is supposed to (and does!) do what it says.

Now a few saved web pages involved with WordPress tips and my Add-ons Page are hijacked by Yahoo search results.

I've tried setting the default search engine to Google (successful) and doing the about:config change, but this takeover still happens.

I can't uninstall the YouTube thing as my Add-ons page stays for only 2 seconds! CleanMyMac can't do an uninstall as the program does not show up.

All other stored web pages are unaffected until today when I clicked on a web site and got instead Yahoo search results for the website's name!!!

Is this a virus??

Can anyone help?

It's not a virus. Uninstall CleanMyMac, it's useless. I'd also say get rid of Firefox and use Safari, but if you want to keep Firefox, you'll have to uninstall the add-ons somehow.

I would try resetting your profile by deleting profile files.
Look at this (look at the Mac OS X section):
Profile folder - Firefox - MozillaZine Knowledge Base