Firebug says "No Javascript on this page", even though JavaScript does exist on the page
Solution 1:
As of Firefox version 50, it seems that Firebug will no longer work as Mozilla are migrating to Firefox Devtools. The tab Tools/Web Developer has a 'debugger' option. See this page for a discussion on the migration. Read all the posts dated 19 November for further information.
It seems to me that the option gives less functionality but Firebug per se is never going to be fixed for Firefox 50 and beyond
Solution 2:
EDIT 2017 April
This answer is now outdated and I'm not even testing it anymore because I'm no longer using Firebug (a shame, I loved it years ago in a time it was The tool of trade for web developing).
EDIT 2014 November
The original answer is now more than a year outdated due to firebug evolution. Thanks to @AmadeusDrZaius to keep it on tracks
Answer for outdated firebug versions For anyone still with this problem after updating Firebug. Just click Clear Activation List option inside the Firebug menu.
For those who can't find it, note that "Clear Activation List" shows up in the dropdown from the Firebug icon in the toolbar, but not the Tools > Web Developer > Firebug menu.
I guess due to the bug previous hitting your working page it stays listed as "no activate FB for this site please".
Solution 3:
I encountered No Javascript on this page after upgrading Firefox to ver 50.0, with Firebug ver 2.0.18. I tried refreshing Firefox, even completely uninstall Firefox and installed again. Nothing works for me.
A couple of hours later, I downgraded Firefox to 49.0.2, and voila, it works right away. I downloaded the old version here.
Solution 4:
I tried everything on this list and nothing worked for me except resetting Firefox like so:
https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-problems
Solution 5:
I realize there are already 10 answers to this question, but in my case none of them helped. I'm running Firefox version 39.0 and Firebug version 2.0.11. The only fix is to completely restart Firefox (i.e. not just the tab or instance with the problem).
Perhaps it's due to Firefox's over-usage of memory, or maybe there's a bug in Firebug. I can't speak to that, but what I do know is it didn't work. I killed Firefox, opened the exact same page again with no changes made and now it works. This has happened to me numerous times and a complete restart always fixes it, but that's the only thing that does.