Hi Sebastian, thanks for your answer!
In fact I wasn't using a "clean" Firefox profile: I created a brand new one, and I tested this way:
On
https://getfirebug.com/firstrun#Firebug%201.12.6I searched twice for "firebug" with with "Search within response bodies" and the first occurrence in response body was selected ("<title>Firebug</title>").
I unchecked the option and pressed "Forward": it still searched for response body.
I pressed "Back" and nothing happened (it neither freezed).
Looks like there is a different (not freezing) glitch :)
Can you verify it?
Thanks!
Massi