Most people know by now that Firefox Quantum has landed in mobile, and brings the same speed improvements as on desktop. What’s even newer (I think) is that at least Firefox Beta for Android includes a Custom Tabs implementation, compatible with Chrome Custom Tabs. So if Firefox is your default browser, and an app includes custom tabs support, then it will open links in Firefox custom tabs.
The Firefox custom tab feature opens quickly, about as quick as Firefox Focus, and is light enough on memory usage that on my phone with 1GB of RAM, it does not push the app that opened it out of memory. Even better, it seems to “warm up” the full Firefox app, so that opening Firefox from a custom tab (if you need something like opening a desktop site, or printing, or reader view) happens quickly. It also appears to use whatever extensions you have in full Firefox, as long as they can operate in the background without any UI. For me, this includes, most importantly, uBlock Origin, and DecentralEyes, which speed up browsing even further. Also, custom tabs use Gecko, Firefox’s renderer, which for me tends to do better than the system WebView (based on Chrome) for laying out and scaling text of pages that are not perfectly designed for mobile. Firefox Focus uses the system WebView.
There are some things that are not perfect, though. One problem with Firefox on Android is that there is sometimes a bit of a stall after the app is loaded, but before it really starts loading a page. This happens with custom tabs, too, which is not great. It may not be noticeable on faster phones than mine. Also, not all apps support the custom tabs API. The ones I mainly open links from, a Mastodon client and an RSS client, do, but some others don’t. With my default browser set to Firefox, these open links in full Firefox, which has the problems I’ve mentioned before.
Another thing that is more of a preference is that Firefox Focus doesn’t keep history, cookies, etc., essentially always operating as a private tab. Firefox custom tabs don’t seem to, so they’ll use your general Firefox environment. You may or may not want this.
Anyway, for me this means one fewer app installed, which is good in general. But I’ll have to see if the lack of support in some apps for custom tabs is a problem for me.