If you experience a reproducible Firefox performance problem, please consider using the Firefox profiler add-on [1] to record a profile and file a bug with "[qf]" to the whiteboard field. These "[qf]" Firefox performance bugs get reviewed by engineers twice a week. Having a profile makes the bugs much easier to diagnose.
I think that Chrome also suffers on this front? But it's better at doing pre-fetching than Firefox is
This could really just be that part. I have a hard time imagining explicit sabotage of FF on the gmail frontend. The likeliest explanation is that perf testing and the like only happens in Chrome
[1] https://perf-html.io/docs/#/