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