Compatible with Waterfox! It's very easy to use and the interface is nice. Editing doesn't allow to change the agent icon, but the strings may be freely modified (you may input a Safari string in an IE option). It has a site filter, but it's global, not individualised for each agent (that is, there can't be different UA for different sites).

No option to whitelist sites

Leider kann ich die App nicht mehr nutzen. Von Firefox wurde sie deaktiviert und nun behauptet Firefox bei der Neuinstallation, die App sei beschädigt. Was kann ich tun?

good

Show an old version of Firefox which block me on some website as slack.

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Indique une version de Firefox dépassée qui me bloquait sur certains sites tels que slack.

This works just fine on non-mobile devices. I have not tested it on mobile yet, but my review is based on my desktop/laptop experience. I have seen questions like "Why does this add-on need the user's username and password?" when pointing to the permissions requested, that ask for all browser and input data. The way this add-on works is it creates a brand new request on your behalf using the data you input, as well as the data on the website. Don't be alarmed about it, it is not storing your data. Some people who write reviews don't know how things work, don't know why certain permissions are requested, and make bad assumptions. Enjoy the app on, at least, non-mobile platforms! :)

Seems like it might work, have not had a real chance to test it. But to the developer, can you make the Option button much smaller (say about 1/4 the size) as it takes up a lot of room on the toolbar. Or make the options when we want to use it available on the Tools dropdown ? ? ? Easier to get to when we need it ... Thanks

Feature "Edit Curretn User-Agent String or Copy it to Clipboard" don't work on Firefox 52.9 ESR.