Hi ofinalee, I'd try starting Firefox in [[Safe Mode]]. If you don't have the issue while all of your add-ons, extensions, and themes are disabled, you can try adding them back in one by one until you find the culprit. You should look at the [https://support.mozilla.org/en-US/kb/Troubleshooting-extensions-themes Extensions and Themes troubleshooting guide ] and the [[Troubleshooting plugins]] article as well. You could also try [https://support.mozilla.org/en-US/kb/Managing-profiles?s=create+a+new+profile&r=2&e=es&as=s#w_creating-a-profile creating a new profile]. You could also take a look at our [https://support.mozilla.org/en-US/kb/Is%20my%20Firefox%20problem%20a%20result%20of%20malware?s=malware&r=0&e=sph&as=s#w_how-do-i-get-rid-of-malware Malware] section to see if you might have missed some tools that could be useful. Hopefully this helps!

You may need to reset Firefox's default Search engine, [https://addons.mozilla.org/en-US/firefox/addon/searchreset/ https://addons.mozilla.org/en-US/firefox/addon/searchreset/] can help with that. Download and Install MalwareBytes Anti-Malware, run a full Scan. [http://www.malwarebytes.org/ http://www.malwarebytes.org/]. This helps check that there are no viruses on your computer causing issues. You can uninstall this program after you clean off any infections. If you open Firefox, and hit F11, does that change anything?