Not GP but same situation. Firefox isn't always great. Start-up time is really slow, hogs a lot of memory, unnecessarily slow and sluggish and most sites are primarily engineered for WebKit/Blink -based browser engines these days.
Lot's of memory? Firefox nowadays uses less then Chrome (Chrome one process per tab is a killer for free RAM).
As for websites engineered for WebKit/Blink I haven't seen that yet, but I was afraid it might happen eventually - new IE6 (or was it IE5?) will happen again :(.
I've never had an issue with memory (have RAM to spare) even though I use tab-groups extensively and frequently have 600+ tabs stored that way. But I do have issues with slow startup times. Is it linked to tab groups? I didn't see any significant speedup upon switching to an SSD, so I'm assuming it is inherent in the program and not anything specific to reading profiles from disk.