Firefox leaks memory. When I have it open for long time the memory usage will increase and even if I were to close all tabs & do memory minimization it will still use multiple gigabytes of memory.
While I cannot be sure, I assume that is the cause of the general slowdown I experience as well.
Restarting Firefox will fix free up the memory & fix the slowness. I do still use it as my primary browser despite these issues.
Is that on Windows? I use Linux and Mac and haven't had memory leaks. In fact, Chrome is more a problem for me on the Mac since my wife likes to keep Chrome windows open on her profile indefinitely, and Chrome keeps downloading updates on the background and takes a large chunk of my hard drive keeping every version that ever existed in the .App file.