It would be real sweet if you could paste in an instance into the community search, and see what communities they have set up and join straight from the https://lemmy.world/communities community browser UI.
I just found out about https://feddit.uk/ earlier, and being a Brit wanted to join a bunch of the communities and had to use their feddit.uk community browser and then paste the link in one by one into lemmy.world search, and the open up the community in lemmy.world and then press subscribe.
Or even being able to subscribe an instance could be decent, but no doubt a lot more work and resource usage constantly federating new communities across etc.
I found that extension you mentioned and unfortunately can’t find a repo for it to start having a little look myself. I’ve messaged the dev on it to see if he’s interested in lemmy or open sourcing etc. EDIT - I found this version that’s open source and has a higher rating than the other one https://addons.mozilla.org/en-GB/firefox/addon/mastodon-simplified-federation/