Are browser extensions interchangeable?

Question

Browsers

The question is how difficult it is to write and maintain an extension if you want to cover all more or less popular browsers.
The fact that extensions from Mozilla will not suit Chrome and vice versa is understandable. And within one "family" based on one engine, how? Chrome, Chromium, Yandex, UC, Opera, Vivaldi, etc - is at least something compatible with something?
And if not, is there a difference only in the design (that is, all the main code is the same, just write a different wrapper or manifest) or are there really serious differences?

Answer 1

Depending on what the extension does, there can be a lot of common code. For example, I once wrote an extension for ff and chrome, and only a few lines differed there. There was more trouble with the manifesto than with these lines.

Answer 2

And within one "family" based on one engine, how? Chrome, Chromium, Yandex, UC, Opera, Vivaldi, etc - is at least something compatible with something?

In general, Opera (not presto, chromopera) has its own extensions and is not interchangeable with chrome, they do not fit each other and it has its own repository. So even within the same engine there can be differences, and the engine itself has nothing to do with it. For a fork of Firefox called Palemoon, they have their own add-ons that are different from Firefox's.
By the end of the year, the current XUL-based extensions for Firefox will generally be replaced by those written under WebExtensions, Firefox will become incompatible with old add-ons, but it will allow you to install them from chrome, as now, for example, can be done with the Chrome Store Foxified add-on? giving the ability to install a chrome add-on in Firefox.
So the question is too general and Stalker_REDwrote correctly: in "Depending on what the extension does". In chrome, EMNIP, without finishing, as in Chinese chrome, there is no way to create a new panel or change the behavior of standard browser components. Accordingly, such add-ons for chrome are impossible in principle, unlike Firefox