I realise this is a minor issue but it drives me mad and is the only reason I don't use Firefox as my main mobile browser. I have several bookmark folders and sub folders. When I open my bookmarks on Firefox in Android it always defaults back to the top level. So if I'm looking at various news sites for example, I have to click through the folders every time.
No other browser I've used on Android so far does this, they all remember the last folder used except for Firefox. I should imagine it's something very easy to fix.