After switching user agents I wasn't able to get this add-on working. The menu is over complicated and not clear at all. Also, it's impossible to know weather you have changed or not the user agent because the icon or the menu doesn't tell you at all.

User-Agent Switcher by ntninja is way clearer, and worked at first try. You can know at any moment whether there is user agent switch or not by just looking at the icon. That add-on also works on Firefox Android.

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Why is this extension spoofing youtube when it is not set to do so?

Bizarrely over complicated. The ntninja switcher let's you just pick a user agent and go. Never got this one to work.

It works regularly in Mozilla Firefox 129.0B9 X64. It is a pity that manual editing of a list of comparison of domains and UA is carried out only in JSON format, and not in something more accessible to non-specialists.

I started to get blocked by websites as a bot while using this addon. Do not recommend.

This doesn't work at all, with any configuration, and hasn't seen an update in over a year. Version 0.6.1.1 is available on other extension stores, and I would package and install this myself, but there are no releases on GitHub.