IMHO if you want to "protest" the browser monopoly, use something more like Dillo or NetSurf. They have no JS, so web apps are out of the question, but work well for the document-centric sites.
One of the machines I use is an old Dell Latitude D620 (2006, Intel Core2 CPU T7200 2.00GHz[0]) on a debian base. I can tell you from personal experience, on this machine, that Mozilla's latest Quantum Browser is heads above Chrome and their "clones".
It is so quick to load (even loaded with extensions), it replaced Pale Moon as the default browser for this laptop. I was stunned at the performance improvements in Quantum. I remain surprised, every single day, that the modern web has been made available on a laptop that's 13 years old.
All other Chrome-like browsers (Chromium, Opera, Vivaldi, etc) take much longer to load, cause the cpu to throttle (fans kick in) which can lead to overheating and shutdown issues. They are essentially unusable on that machine. Firefox Quantum, otoh, is snappy on this machine.
I say this because I'm genuinely curious by the "too big" comment. I honestly find it puzzling (unless you're comparing a modern, full-featured browser to Dillo or Links2/GUI: both of which are favs of mine for their use cases).
I had turned my back on Firefox for a few years because of performance issues, caching problems. They didn't just correct these issues, they've done so in a way few software upgrades have ever managed to do in my experience (dating back three decades) and have blown past their competitors from a purely performance standpoint.
[0] https://www.cpubenchmark.net/cpu.php?cpu=Intel+Core2+Duo+T72...
You may be experiencing OS caching. If you normally use Chrome, Chrome is already cached in memory by the OS and will open faster than something you haven't started recently.
> On my MBP it also makes the fans spin up.
I've noticed that Chrome tends not to do this while still being slower. This may be related to the way it uses more processes, which consequently requires more resources per operation (and thereby higher visible latency) but splits the load better between cores so that no individual one gets hot enough to require additional cooling.
In theory this could be "better" if the browser actually needed to use 8+ cores for some reason, but in general if that is happening it is some kind of anomaly or defect (like some adspam mining Bitcoin in javascript) rather than any normal behavior. And Firefox does use other threads/processes for most of the few things that actually benefit from it.
But yes, the loss of EdgeHTML (following a few years after the loss of Presto) is a sad thing for the health of the web.