Do you see any error messages in the Browser Console?

• https://developer.mozilla.org/Tools/Browser_Console

No, I don't. I checked all items in all submenus I could (I mean "Net", "CSS", "JS" and so on), but I only see some error messages from site's CSS and a lot of other info, but nothing of it seems related to developer tools itself. When I click on "responsive design mode", nothing bumps up in the browser's console.

(I'm certain that I'm viewing namely browser's console. I opened it using Ctrl+Shift+J)

Does it make a difference whether multi-process is enabled or disabled?

See the "Help -> Troubleshooting Information" page under Application Basics: Multiprocess Windows for details.

You can disable multi-process windows in Firefox by setting these prefs to false on the about:config page.

• browser.tabs.remote.autostart = false
• browser.tabs.remote.autostart.2 = false

You can open the about:config page: via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

• http://www.ghacks.net/2016/07/22/multi-process-firefox/

It says "Multiprocess Windows 0/1 (Disabled by add-ons)" and it doesn't change when I toggle both browser.tabs.remote.autostart settings. Doesn't affect the initial issue as well.

I reinstalled the Firefox, the problem remains.

Maybe it's somewhere in the profile folder. Where do devtools store their cache, state or something?

I've noticed that when I press Ctrl+Shift+M the scrolling bar becomes "mobile" (thinner, trasparent, round borders). Nothing else changes, though. And I can't revert it pressing Ctrl+Shift+M again. Even page reload doesn't work.

UPDATE: I created new blank profile and there devtools work properly. The question is now how do I make them work on my existing profile.

I found some kind of solution.

Examining devtools* setting in about:config I've noticed that settings' names in new profile differ from the ones in the old one. In particular, there is no settings named devtools.responsiveUI.*, except of one. I deleted all those settings in old profile and now it works!

Basically, the problem is responsiveUI version in old profile and in a new one somewhy differs significantly, and Firefox's update didn't help with it (although it should, as devtools are core part of Firefox itself (as far as I know) and should be updated for all profiles). For example, in old profile I still don't have network throttling tools etc.

Note that there are two different RDM versions. The 'classic' version (black background) that is used when multi-process is disabled and a new version that is used when multi-process is enabled and that is centered and has a white background and has the device selector. I don't know what prefs are used by both versions, but there could have been a conflict.

I normally only see scroll bars when I drag the mouse in the location where the scroll bar should be.

That makes sence. I want to test it. How do I enable/disable multi-process?

You can check "Multiprocess Windows" to see the current state.

• "Help -> Troubleshooting Information" -> Application Basics: Multiprocess Windows

When multi-process is enabled and the first number is not zero (1/1) then you get the new design. You can disable multi-process by toggling the opt-in pref to false on the about:config page with a double-click to see the old design.

• browser.tabs.remote.autostart.2 = false

You need to close and restart Firefox to make the change effective.