Are you talking about Firefox or your computer? When you are on the site, are you actively doing something, or doing something like downloading or using a web camera? Is this with many websites, some, a few?

Usually on FB, making comments and viewing vids.

on pc. But whichever site, it will slow to be unresponsive after 2 hrs.

Many site issues can be caused by corrupt cookies or cache. * Clear the Cache and * Remove Cookies '''Warning ! ! ''' This will log you out of sites you're logged in to. You may also lose any settings for that website. * '''Cookies; ''' [https://support.mozilla.org/en-US/kb/delete-cookies-remove-info-websites-stored Delete cookies to remove the information websites have stored on your computer] {web link} * '''Cache; ''' [https://support.mozilla.org/en-US/kb/how-clear-firefox-cache How to clear the Firefox cache] {web link} If there is still a problem, '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''' {web link} by holding down the '''<Shift> ''(Mac=Options)''''' key, and then starting Firefox. A small dialog should appear. Click '''Start In Safe Mode''' (not Refresh). Did this help? While you are in safe mode; Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches. ''[https://support.mozilla.org/en-US/kb/forum-response-disable-hardware-acceleration How to disable Hardware Acceleration]'' {web link}