I already asked on IRC but I prefer that there is a discussion about this.
In Nightly/dev edition there is the new responsive view but it’s missing the chance to set a custom user agent, so I need to use an extension for it.
An example I tested with the useragent of Facebook in-app browser that is not a standard useragent.
The only way actually is use the pref to get back the old view but is only a temporary workaround.
Thanks for bringing this up! I would like to revive some form of UA control in the new RDM, but for the moment it’s been lower priority.
I am curious, what tasks do you need UA switching for? Do you have to have the ability to enter an entirely custom UA, or would a list of various other browser and mobile UAs be sufficient?
I think that a custom one it’s the best way, when is required debugging you can try with switch with the mobile ones, but when it’s a case specific like mine you need the right user agent string.
Can you share more about your specific use case? That may help us build the right tool.
I guess only a text input where it is possible to paste the specific user agent is the simplest way. When is required a debug with a specific user agent i need only this things to debug.
Why not use the User-Agent Switcher add-on? I use it for watching Netflix on Linux (because Netflix has a User-Agent whitelist which still does not include Firefox on Linux although Firefox works perfectly). The addon also let you insert whatever string.
If this really should get into DevTools then one nice way would be within Creating a new Custom device (which is not possible yet, but I’ve seen a bug for this on bugzilla somewhere). Then you’d also have it persisted and reusable. @Mte90 what you think?
(Disclaimer: I’m not a devtools developer, just a user)
I am already using an extension for switch the user agent as I already said but that feature there was on the devtools so this was my point 
Fair enough, sorry, I must have missed that
What you prefer the “old” way (before FF 52) of having a direct text input or the one I described above (via adding a persistable custom device which can have some custom user agent string, too)? Asking because - as a fellow devtools user - I’d prefer via adding custom devices as this makes settings reusable latter and also it would be less UI clutter on the RDM frontpage.
In my idea a custom user agent string is the most simpliest way to have the feature right now but, yes the chance to save it in the browser it’s very interesting.
I went to go use this feature after moving to FF from Chrome (over privacy concerns). The use case which brought me here - was when I was trying to debug which uses UA sniffing. Both Safari and Chrome have this feature pretty easily available.
In Chrome you can access it using the responsive browser feature - and Safari under the Develop > User Agent menu.