I'm working on a pretty nasty (legacy and poorly optimised) but otherwise still rather "normal" website at work.
Opening the dev tools makes Firefox hang for almost a minute, I suspect this is due to some issue with source maps, thousands of source files and large (several megabytes of) minified code.
The debugger often reports _wrong_ values on hover especially in useEffects whereas console.log shows the right one.
Other than the js debugger I have 0 problems with the Firefox devtools, it often spearheads features that I use on a day-to-day basis like highlighting grid layouts. Super nice!