You are describing is the computers CPU being overworked. Try to keep track of what you were doing and where. '''https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently''' '''https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster''' '''https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix''' '''https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding''' '''https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding'''

Thanks, you're probably right, though I don't know why the problem suddenly started occurring when it didn't before. It may be a plug-in. I'll try to keep better track of exactly what I am doing when Firefox hangs.

You can check for problems with the <b>places.sqlite</b> database (bookmarks, history) in the Firefox profile folder. *https://addons.mozilla.org/firefox/addon/places-maintenance/ See also: *http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox *https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file 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

Thanks - I ran places-maintenance & found no problems but will try changing profile.