A Firefox “Profile” contains all the modifications you made to the original Firefox installation including Bookmarks, Themes, and Add-ons. Most Themes and Add-ons perform perfectly, but there are some that cause memory leaks, browser hangs, slower performance and overall stability issues. There is a official sheet listing some of the known offenders called “Problematic extensions”. Some Add-ons uninstall properly, some don’t. Typing about:config in your browser after uninstalling an Add-on and searching/sorting by name may show remnants. Its not clear if those remnants default control back to the browser, or not, so you may still experience performance issues.
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