Hi, try changing Firefox updates to 'Check for updates but let you choose to install them.' - 3-bar menu > Options (General) Firefox Updates. That way, you will be informed but updates won't download automatically.

If you want to see what's happening, look in the PCs Task Manager (Ctrl+Shift+Esc). In the Performance tab, Resource Monitor is very helpful.

For slowness generally, you can try reducing the number of processes: Go to the 3-bar menu > Options (General) - Performance, then deselect 'Use recommended performance settings' and change 'Content process limit' to 1 or 2. See - Firefox's performance settings, and Startup, home page, tabs, and download settings.

Whilst in the 3-bar menu, you can also go to Options (Privacy & Security) - Permissions, and tick/check 'Prevent accessibility services from accessing your browser' then restart Firefox. More - What is the Accessibility Service Indicator?

Firefox might not run as smoothly, but if necessary, you can try turning off Multiprocess altogether: Type about:config into the address bar, press Enter, accept the warning, scroll down the alphabetical list to browser.tabs.remote.autostart and browser.tabs.remote.autostart.2 then double click on each to change 'Value' from True to False and restart Firefox.

More about Multiprocess - https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox

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Firefox is only updated once in a while.

Just in case; You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

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). Is the problem still there?