Try these one at a time, checking after each.
Go to FF menu > Help > More troubleshooting information > click 'Clear startup cache' button on the right > restart FF
Clear the browser cache:
- press Ctrl-Shift-Delete (Mac: Cmd-Shift-Delete)
- set 'Time range...' to 'Everything'
- untick all items except 'Temporary cached files and pages'
- clear, then restart FF
See what happens in Troubleshoot mode (https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-...). If it works better, see 'Turn off hardware acceleration' below. If that doesn't help, in normal mode disable all your add-ons > re-enable one at a time and test. If you have a lot of add-ons, disable half of them. If the problem still exists, re-enable them, disable the other half, and narrow it down in a similar fashion.
Via the address bar, go to about:config > search for accessibility.force_disabled > change the value to 1 > click check mark > restart FF
Turn off hardware acceleration:
- go to FF Menu > Settings and enter 'hardware' (no quotes) in the search box
- uncheck 'Use recommended performance settings' > uncheck 'Use hardware acceleration when available'
- restart FF
Disable Efficiency mode: in about:config, change dom.ipc.processPriorityManager.backgroundUsesEcoQoS to false > restart FF
Disable VPN if you're using one
Change proxy settings: FF menu > Settings > search for proxy > click Settings button > try the other settings
Change DNS settings: FF menu > Settings > search for dns > try the other settings including another provider
Re the taskbar: known old bug with no solution yet. Try this:
Type about:config in the address bar > press Enter
- click 'Accept the risk and continue' if that appears
- search for widget.windows.fullscreen_marking_method > change the value to 0 or 1
Make sure to click the check mark if there is one after making the change.
