For me it's the weird new way of handling custom search engines that annoys me. Old workflow: tap the address bar, start typing your search term and then either press return to search with default search engine, or 2) tap one of the custom search engine buttons. In both cases, the search was executed directly.

New Firefox: (go to settings to enable non-default search engines to be displayed) Best case: tap the address bar, enter your search term, press return or one of the non-default search engines and then return.

The problem is that after adding a custom search engine, it ends up at the bottom of the list, meaning that in order to select it after typing my search terms, I have to scroll down, causing the keyboard to collapse. After selecting the right search engine firefox doesn't search automatically but rather I have to tap the address bar again and then press return. So, where in the past I had a "tap, type, tap, (search results)" experience, now it is "tap, type, scroll, tap (bottom of screen to select SE), tap (top of screen into the address bar to get the keyboard back), tap (bottom of screen [return key on keyboard]), search results"

I very much would like to go back to the old behavior.