The current version of FireFox is a memory hog. In Windows, if you do a ctrl alt del and start the Task Manager and then go to Processes and click on Memory, you can see how much RAM Firefox is using. More than likely, it is the lion's share of your RAM. If you shut down Firefox and restart it, yoou will see the amount of RAM used has dropped significantly.
When Windows is memory challenged, it starts using virtual memory in the form of hard disk space. The access time for virtual memory is much longer than for actual RAM so Firefox and any other operations slow way down. It's the price we pay for having Firefox remember every thing you have done since the last startup plus what you had for breakfast .
I made the mistake of updating Firefox on an older laptop and Firefox slowed to less than a crawl. I am running 12 GB of RAM on my computer and astill have to shut down and restart Firefox on a fairly regular basis. The only solution was to revert back to an older version.