Just found this. I do not use Firefox but after reading other forums that are discussing the same/similar problems, I would do as this says. If it does not sort the problem I would be doing a clean install of the latest version of Firefox and clean install of the blocker.
(1) Open Firefox's Add-ons page using either:
Ctrl+Shift+a
"3-bar" menu button (or Tools menu) > Add-ons
in the Windows "Run" dialog, type or paste
firefox.exe "about:addons"
In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".
In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. All extensions are optional; none come with Firefox.
Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.
Any improvement?
(2) You can search for remaining issues with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.