It'll compress to about 30% of the size, it's easy to do, and it shouldn't add more than a tiny CPU overhead over formatting the JSON itself.
It solves half the problem with like 15 minutes of work.
Then an hour of writing good tests.
Then lots of manual and automated testing on four or five platforms, and fixing the weird issues you get on Windows XP SP2, or only on 64-bit Linux running in a VM, or whatever.
Then making sure you don't regress startup performance (which you probably will unless you have a really, really slow disk).
Then implementing rock solid migration so you can land this in Nightly through to Beta.
Then a rollback/backout plan, because you might find a bug, and you don't want users to lose their session between Beta 2 and Beta 3.
Large-scale software engineering is mostly not about writing code.
Add the code that's able to load compressed session backups and leave it in for a couple versions.
Once enough versions have passed enable the code that writes compressed session backups.
It's really not that hard to do unless you want to enable it now.
No, for example, LZ4 is unbelievably fast:
https://github.com/Cyan4973/lz4
almost 2 GB per second in decompression!
I've just tried compressing some backupXX.session file (the biggest I've managed to find, just around 2 MB) and it compressed to 70% of the original, probably not enough to implement the compression -- and I suspect the reason is that the file contains too much base64 encode image files which can't be much compressed?
So the answer to having sane session files can be first to stop storing the favicons (and other images(?)) there? I still believe somebody should analyze the big files carefully to get the most relevant facts. For the start, somebody should make a tool that extracts all the pictures from the .session file (should be easy with python or Perl, for example), just that we know what's inside.
For a while now I have been running a cronjob to commit my profile's sessionstore-backups directory to a git repo every 5 minutes.
This is because, occasionally, when Firefox starts, it will load tabs in some kind of limbo state where the page is blank and the title is displayed in the tab--but if I refresh the page, it immediately goes to about:blank, and the original tab is completely lost.
When this happens, I can dig the tabs out of the recovery.js or previous.js files with a Python script that dumps the tabs out of the JSON, but if Firefox overwrites those files first, I can't. So the git repo lets me recover them no matter what.
What I have noticed is that the git repo grows huge very quickly. Right now my recovery.js file is 2.6 MB (which seems ridiculous for storing a list of URLs and title strings), but the git repo is 4.3 GB. If I run "git gc --aggressive" and wait a long time, it compresses down very well, to a few hundred MB.
But it's simply absurd how much data is being stored, and storing useless stuff like images in data URIs explains a lot.
Like you, I also observed that exactly the people who depend on the tabs to "remain" after the restart are those who are hit by the bugs in the "restoration" and as I've said, I believe the users would more prefer to have "stable" tabs and URLs than the "fully restored sessions in the tabs" when all the tabs fully randomly (for them) disappear. Maybe saving just the tabs and URLs separately from "everything session" would be a good step to the robustness (since it would be much less data and much less chance to get corrupted) and then maybe, pretty please, an option "don't save session data" can be in the about:config too)?
Once there's decision to store just the URLs of the tabs as the separate file, the file can even be organized in a way that just the URL that is changed gets rewritten, therefore making the "complete corruption" of the file impossible and also removing the necessity for Firefox to keep N older versions of the big files (which then eventually still don't help the user like you).
Yes, that would be very, very useful. I can get by if the tab's scroll position and favicon and DOM-embedded images--and even formdata--are lost. But if the tab itself is lost, and it was a page I needed to do something with, I may never even realize it is gone...
Also, what do these add-ons do? The only use case I can think of is figuring out whether the user has a tab open to a given site, and that's going around the browser's security model, so breaking that would be a good thing.