Firefox

You can see the code of extensions, but it may be minimized, so it hard to known what the code do.

Extensions with label "Recommended" are pass the manual review of Firefox moderators, so you can trust them more than addons with no this label. However you still should keep in mind that any extension developer may be victim of complex scam attack.

The most probable reason usually is a not enough funding the developers

• Developer spend time on maintaining the project but users does not donate them
• Scammers offer to developer some integrations that not looks too suspicious and allow them to earn some money
• Developer agree offer and after some time scammers enables malware to hack extension users

To minimize the possibility of hijacking addons by scammers, we have to:

• conduct background check before install extension
• ensure the extension have github with open source code and developer are real person
• ensure development are active and developer have high engineering skill, check them respond on feedback and issues
• donate the developer if you like the product, to motivate them keep distance of scammers offers

You can open/extract extensions (at least Firefox ones) as zip files, they'll contain the code and assets used by the extension.

From what I've experienced with my extension, every update has to go through a review process. Firefox is pretty fast, chrome takes a few days and edge takes a while (opera hasn't finished reviewing my first version, so I stopped trying with them).

The only time I failed a review was early on when my build script choked and I submitted an empty file to Chrome (whoops). So I can't really tell how good those reviews are, but I'm not planning on testing them. I just know that they exist