Early on Firefox felt fast for sure (on most platforms anyway), but at some point around 4.x or 5 I wanna say it started slowing noticeably, and it would continue on that path until Quantum or thereabouts.

On OS X though Firefox always felt a noticeably more laggy and clunky than any other app it might be running alongside — something about the Mac implementation of XULRunner wasn’t optimal and it showed… even if you painstakingly skinned Firefox to match OS X perfectly visually, it still felt kinda slow and clunky. That’s why Camino, which was a Gecko browser with a Cocoa/AppKit UI came into existence, and it always felt a good deal more responsive than Firefox did.