Recently my Firefox got updated to version 51.0.1. Initially it was showing some reminder check (I'm not clear with that) related to Firebug, but I ignored it. Now, the Firebug toolbar button is always shown deactivated.
I've reinstalled Firebug several times, but the result is always the same.
Is there any compatibility issue? How do I activate Firebug?
There is a solution to use firebug [version 2.0.18] in latest version of firefox [this time 51.0.1]
To enable old Firebug
In address enter about:config, find
browser.tabs.remote.autostart set it false
browser.tabs.remote.autostart.1 set it false
browser.tabs.remote.autostart.2 set it false
This is the new version of FF and it is not working as intended and making life of developers & designers even more difficult... look at review https://addons.mozilla.org/en-US/firefox/addon/firebug/reviews/?page=1.
This actually is FireFox DevTools as FB is no longer supported for future developments
You can install the previous version to work with, I installed it and my problem is solved i have my old version back.. https://addons.mozilla.org/en-US/firefox/addon/firebug/versions/.
New version of comes with some new features but some handy features are also left out. It looks totally new to me. May be one day i have to get used to it. For now one can revert back to old version
This is because Firebug is integrated into the Firefox DevTools once multi-process Firefox is enabled and it doesn't track the enabled state of those tools. This is a known bug.
Note that a click on the Firebug button with multi-process Firefox enabled actually opens the Firefox DevTools with a Firebug theme trying to make the transition a bit easier, because Firebug is officially discontinued.
If you want the old Firebug back and by that make the Firebug button work again as expected, you need to disabled multi-process Firefox as explained in an answer to a similar question by going to about:config and setting the preferences browser.tabs.remote.autostart, browser.tabs.remote.autostart.1 and browser.tabs.remote.autostart.2 to false. Though note that this is just a temporary workaround, because multi-process Firefox will be mandatory at some point.
Furthermore, the Firefox DevTools offer a lot more features than Firebug but are also lacking some Firebug had. Therefore, a bug is filed to close those gaps.
In the answer to a related question I've listed what your alternatives are.
If you want to stay with the Firefox DevTools, there is a migration guide for Firebug users.
Firebug seems to have activated for me after I got the Web Developer Add-On on Firefox, link: https://addons.mozilla.org/en-us/firefox/addon/web-developer/
Open the default Element Inspector, and click settings icon
Under the Themes section you will see Light option is checked and change it ot Firebug automatically Firebug will activated.
The best way to install firepath and firebug to downgrade firefox by downloading older version of firefox like 31 from firefox ftp.
I downloaded older version of firefox from http://mozilla_firefox.en.downloadastro.com/old_versions/
After downgrading go to option-> Update and check "Never check for updates" checkbox
If the below setting has been made:
In address enter about:config, find
browser.tabs.remote.autostart set it false
browser.tabs.remote.autostart.1 set it false
browser.tabs.remote.autostart.2 set it false
Then reset firefox by typing 'about:support' in the address bar
Click "reset firefox"
By doing this you should be able to install firepath, firebug addons from the mozilla addons page
来源：https://stackoverflow.com/questions/41929647/firebug-toolbar-button-is-always-shown-deactivated-since-firefox-51-0-1