Firefox

Everything a computer does uses resources so every extension will have some performance impact, though you usually won't notice the difference from any one extension. As a general rule of thumb, the more an extension does the more resources it needs and so the bigger the impact. Some extensions free up additional resources (eg by blocking ads or trackers) which may also result in a net performance gain dependant on the page you're viewing.

A large extension on disk doesn't necessarily use more memory and CPU time than a small one, but it is more likely to. The only reliable way to tell which extensions are resource hogs is by benchmarking them.

Disabling extensions not in use is a good idea, though personally I'd uninstall them instead.

Enter about:processes in the address bar.

There's a speedometer icon next to the extensions row which will monitor resource usage for 5 seconds. There's a wealth of information.

checking if the extension is obfuscated is a indicator if it might be doing something shady/malicious. Sadly some build systems like webpack/react produce pretty bundled code that looks pretty obfuscated too ... so it's not a perfect rule, just an indicator to look at.