Would you please describe what you mean by slow, including where and how you experience this ? Is this only on video ? Is it still slow with only one tab open ? How does this compare with Internet Explorer ? If a page loads slowly how long about does it take 2 |5|10 seconds, or over a minute ?

Have you seen

• Firefox is slow after updating to Firefox 38
• Quick fixes if your Firefox slows down
• Fix problems connecting to websites after updating Firefox

Are you noticing high CPU use or High memory use ?

Is this support site slow ? Have you tried temporarily disabling your Firewall AV and any other security software and then tried ? Does that improve things ?

You can try to create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox