This can be done from the about:preferences page (also available from ToolsвћћOptions). Thus, to use Firebug in newer versions of Firefox, you will need to disable Firefox from entering multiprocess mode.

Using Firebug in newer versions of Firefox Firefox automatically enabling multiprocess mode will depend on multiple factors, including what other add-ons are installed in the profile. The fact that Firebug does not actually work in multiprocess Firefox will account for different people having different experiences with Firebug working or not working in a particular version of Firefox. You will be able to disable e10s for a while to keep using Firebug 2, but this wonвЂ™t last forever. e10s) and converting it is too complex, so it will stop working when e10s is activated in Firefox. They also direct you to read: Unifying Firebug & Firefox DevToolsДЇirebug 2 doesnвЂ™t work in multi-process browsers (i.e. Firebug will then only assist the user with migration to FirefoxвЂ™s built-in tools. When opened in a multi-process Firefox, FirebugвЂ™s tools cannot be used. This way Firebug can help users to easily migrate into built-in Firefox developer tools вЂ“ in case itвЂ™s running in multi-process enabled browser.

The extension is marked as multi-process (e10s) compatible so, it isnвЂ™t blacklisted and users can continue installing it. However, it also explains that :ДЇirebug 2.0.18 fixes issue 8030. Firebug does not work with multiprocess FirefoxДЇirebug 2.0.18 is compatible with Firefox 30 вЂ“ 52

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