I noticed recently that Firebug was running very slow whilst inspecting elements or debugging client-side scripts. In the past when noticing performance issues in Firefox, a straight-forward opening and closing the browser normally resolved any issues.
What I found after investigating this problem online is that Firebug keeps a record of all breakpoints and sites where Firebug was used. As you can imagine, this will accumulate over time. To remove all history, go to your Firefox profile directory which can be found here:
C:\Users\<Windows User Account Name>\AppData\Roaming\Mozilla\Firefox\Profiles\<Firefox Profile ID>.default\firebug
You will find two files within Firebug folder directory:
annotations.json - contains a history of website browsing.
breakpoints.json - contains currently set breakpoints.
Close all running instances of Firefox and delete both files within the directory.
Judging by posts from other users online, there could be other contributing factors to Firebug's sluggishness at times. Hopefully, by carrying out the steps above you will notice a difference.