T or Browser is built on a basic Firefox base. The developers advise against expanding the browser’s facilities using extensions. Technically, you can add Firefox extensions, but it might be the case that these additions cause a security compromise or malfunction. So, you’ll have to decide on a case-by-case basis whether the risk is worth it if there’s an extension that you need.
The LibreWolf developers aren’t quite as stringent in their advice against installing extensions, but they do recommend limiting your additions to the essentials because the extensions themselves can cause security problems. In actual fact, the risks are probably similar to those of Tor Browser as the two browsers work in a similar way.
Vivaldi allows the use of Chrome extensions, which gives access to an enormous library of add-ons to choose from. The settings page points out that you may run into problems in rare cases when it comes to compatibility, and it includes a link to the official web forum to report incompatibilities.
Brave uses the same approach as Vivaldi as it has a few built-in extensions and then tries to maintain a good level of compatibility with Chrome extensions.
Zen has a two-tier approach to extensions as it allows the addition of standard Firefox extensions and it has its own system of so-called mods that are created by the community. These mods are added via a store page, and at the time of writing there were more than 80 of them, most of them concentrating on UI enhancements. Between the two, there’s quite a lot of features out there for you to add.