That's getting better, for sure. The bad part: on mobile you don't have access to the whole API you have on desktop. For example, on mobile there's no access to your history. (I wanted to do an extension that cleaned older stuff from my history...)
You have to jump through extra hoops, at least. I was able to install my own custom, unpublished extension easily with Kiwi.
Lots of options on Android, and for iOS there's Orion.
Safari extensions work on iOS and iPadOS as well as the Mac.
seems to be a Chrome thing (gee I wonder why)
safari and firefox support them
Firefox on iOS definitely does not support extensions. Switched to Orion and couldn't be happier.
On Android I use a two fisted approach; Chrome for things that require auth/payments, Kiwi for everything else. Kiwi is an open source fork of Chrome, and it allows extensions. Unfortunately it's not up to date to the latest (secure) Chrome, but I accept that because not having control over the browser is its own form of exploit.
On iOS/iPadOS Firefox and Chrome extensions seem to mostly work in the Orion browser.