Outside Chrome, the answer is that the extension support is a very big and fragile (hard to maintain) patch for Chromium.

Here’s a list of browsers with extensions:

• Kiwi and Yandex browser support most Chrome extensions
• Firefox, Mozilla’s Reference Browser and Firefox forks support most Firefox extensions, but you need to make a “collection” if you want more variety than the default list
• Samsung Internet supports some content blockers as app-based extensions
• SmartCookieWeb, Berry Browser, Sleipnir support userscripts
• Many other browsers also have some form of tracker and/or ad blocking