I was having a similar experience earlier this year, so I reluctantly did the same for a while. I got sick of chromium though, really missed reader mode for one thing, so I decided to put in some more effort to troubleshoot.

For me what did it was switching to wayland (sway[1] specifically, from dwm on x11). Now it's a good step faster than chromium.

I agree with the sibling comment; there's something wrong going on, what you're describing isn't firefox's baseline.

[1]: https://swaywm.org/