It's a mixed bag. Ad blocking speeds up some websites substantially. IMO that makes Firefox mobile faster overall.

Kiwi browser also has ublock (and supports adding any desktop add-on) and still does not crash or slow down like firefox does. I'd really like to use firefox in mobile too but with Kiwi being an option, there's almost 0 reason to use firefox.

Vivaldi puts Firefox Mobile to shame when it comes to speed.

Unfortunately, Vivaldi is not FOSS which makes it a non-starter for many.

Brave is FOSS and fills the gap (if you can find all the crypto ads toggles hidden as hamburgers).

I find Brave slower than Chrome on my phone even on sites filled with ads.

Enable JavaScript JIT (disabled by default on Brave for something like a year) to trade some safety for performance.

I frequently have to close all my tabs because the browser has become unresponsive.

This doesn't happen to me. I currently have 60 tabs open, and that's only because I have 'auto close after 1 month' turned on.

thanks for the tip