I mostly use Firefox because it lets you bookmark a page without overwriting an existing bookmark. I have a lot of bookmarks and they sometimes need to be in multiple folders, but Blink-based browsers only let you create duplicates by manually copying a bookmark and pasting it elsewhere.
I'm finding certain security features being lacking from Firefox to be annoying. I should be able to set JIT javascript compiliation and DRM to be opt-in on a per-site basis considering the security risks in those codebases, especially considering the weaker security of the Gecko engine.
I'm also concerned about the prospect of the only browser engine besides Webkit being Blink, but if that were my only issue I could spoof my user agent.