Custom search engines are limited to PC browsers only. Unfortunately, the mobile versions of these browsers will not let you set up a custom search, so you're stuck tapping on the Web view in all your mobile search queries.
You have to visit a different search engine. And while the tab is open it appears in the list of selectable search engines. At least that is how I set up searxng as my default se.
Are there any extensions that fix Google on the mobile version of FireFox? It doesn't have all the same extensions available on PC, where I do have one that fixes Google.
The "web" category thing the article talks about is not an actual fix.
depends on what you mean by "fixing", I guess. I just resorted to not using Google at all, and use uBlacklist to filter out garbage websites from my search results.
The most recent version now includes a lot more extensions, and I think you can get even more with the nightly build. If you really can't find what you need, try one of the Firefox forks, which often have even better extension support.