Iam using zorin on an 8gb ram laptop which shouldn't lag that much and most of the time feel a bit of lag
After some digging and i find a process called "Isolated Web Co" taking nearly a 1gb and half of memory and i don't know from which app does this come from but it starts always with firefox
Yeah i did re-install firefox from the terminal and it appeared to be that the problem is in firefox itself as you said, because i removed all add-ons (extensions) but it didn't help solving the problem either
I didnt experience the same thing on windows so i guessed its a linux issue and the forums i viewed on ask-ubuntu were useless for me so i posted the issue here
So my only option is to move into another browser but not chrome, I already ditched chrome because of its anti ad-block manifest-3