Been wondering the same for quite some time now,
just gut-wrenching when your in-the-moment-freestyle-css’ing with firebug gets blown to bits by
an accidental reload or whatnot….
For my intents and purposes, I’ve finally found the tool…. : FireDiff.
It gives you a new tab, probably some weird David Bowie reference, called “changes”;
which not only allows you to see/save what firebug, i. e. you, have been doing,
but also optionally track changes made by the page itself….if it and/or you are so inclined.
So thankful not having to re-type, or re-imagine and then re-re-type, every css rule I make…
Here is a link to the developer (don’t be disparaged by first appearance, mayhap just as well head straight over to the Mozilla Add-On repository .