I love Firefox, the way it's set up in intuitive ways, and general reliability. I do have two issues.
First, every time Windows updates, Firefox slows down until its next update. My impression is that Microsoft does not like Mozilla. Firefox is a competitor to Edge, after all.
Second, when I initially start Firefox, it takes a long time to load. Sometimes, I just take down Firefox an start it up again, which makes it work better, for some reason.
Edge starts up quickly, BTW, but I hate Microsoft, though I like their Hotmail.
This is primarily because of Microsoft .Net3<, Oracle Java6<, and Python3<. Systems, programs and users each run much better when those components are not tied to any of them. Do you see why my team, a great supporter of Firefox, loves Unix? Java loves to crash all components.
In my experience, all programs slow down quite a lot when updating Windows, not just Firefox.
Don't know about OP's, but maybe he can leave Firefox closed and see that happen with other programs.
One could see in Window's task manager what are most demanding programs on PC's hardware by sorting on the specific column.
Does OP have SSD on their system? Maybe they are resource constrained on a piece of hardware.
Sorting the processes in task manager by the various resource usage columns can help pinpoint the one(s) most constrained/utilized in that moment(Disk, RAM, CPU, GPU).