Firefox has long had an issue with freezing/hitching. Here’s the fix: Go to about:config, search for browser.sessionstore.interval and set it higher than its default (10000). The units are milliseconds (divide by 1000 to get seconds). I prefer to set it to 60000 or greater.
I will grant that I have Firefox on 5 workstations and only one really, really bothered me (my work computer, where I spend more of my computer time). Anyway, if you have the problem here’s the solution that solved mine at work.