I can't seem to add it to my add-ons

It's not installing at all, very stressful

You shouldn't use it if care privacy. Random UA string which is not exist in the wild make you have a unique fingerprint.

A lot of non-exist random UA is generated by this addon: Firefox/110.0esr (now version is ESR 102, next one is ESR 115), Firefox/123.0 (haven't release). The author didn't reply that issue but actively merge
dependence: https://github.com/tarampampam/random-user-agent/issues/415

Maybe it should be as its name, "random", and the author think it okay?

Also the addon will send an extra cookie with base64 (decode is the ua string) which is strongly against privacy, but the author don't consider it as problem, see https://github.com/tarampampam/random-user-agent/issues/420

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不要因为隐私而使用这个扩展，这个扩展生成的版本号是在制定范围内随机生成的，但是制定的规则有问题，生成的几乎都是不存在的火狐版本号、谷歌版本号。给作者反馈也不回应，作者或许是想“我本来就说的随机哪里骗你了？”这个真的名副其实是随机。

第二个问题是，这个扩展会生成额外的 cookie，发送给网站。解码后是随机的 UA，这是严重违反隐私设计的，作者同样不觉得是问题。当然也可以像 Cookie AutoDelete 那样做到不发送到服务端的 cookie，但作者直接关了，见：https://github.com/tarampampam/random-user-agent/issues/420

I have been trying to add this "Random User Agent" in my firefox but isn't working.
Please know what you will do this.
Thank you

This add-on works well + is Open-Source