Firefox is agonizingly slow for me on an otherwise idle system, on my old, slow (2 core, 1.5 GHz) laptop, with only 8 tabs open.
It takes about 30 seconds to load a web page, and I'm using an ad blocker (uBlock Origin) and also block unnecessary Javascript (via uMatrix). I have some other blockers too: Decentraleyes and CanvasBlocker, along with some other extensions like Stylus, Tree Style Tab, and Multi Account Containers which might slow things down as well.
Note that each third-party content blocker you listed slows down page load speed by a fraction of a second on fancy multi-core desktops. That slowdown may be more significant in your case.
It takes about 30 seconds to load a web page, and I'm using an ad blocker (uBlock Origin) and also block unnecessary Javascript (via uMatrix). I have some other blockers too: Decentraleyes and CanvasBlocker, along with some other extensions like Stylus, Tree Style Tab, and Multi Account Containers which might slow things down as well.