The key difference for me was the way they create the names of the links in the popup. In Chrome you need to enter more or less random letters, while in vimperator you can write the link name. The difference in usability was HUGE for me and made me switch to FF for a brief moment from using Chrome since its release day.
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.