The goal of session restore is to restore your session -- your open tabs should come back, the same pages should load, scrolled to the same place, and with the right content.
Someday the pain may motivate me to try to learn enough about Firefox's internals to do it. :)
It would be interesting if somebody would actually analyze what takes the most of the mentioned 300 MB. I see a lot of base64 encoded stuff, if they are "favicons," come-on. There are so many caches in Firefox already, JSON files certainly aren't the place for these images.
But yeah, storing favicon in Session Restore would be pretty bad. I didn't remember that it was the case, though.
https://news.ycombinator.com/item?id=12569999
700 KB of binary images in a 1.7 MB session file, which can be compressed only to the 70% of its size.
I also see a lot of things like \\u0440 which spends eight characters for one unicode character (in another file, not from me). But that file was reduced to 37% of initial size with LZ4. It seems LZ4 is still worth doing, if the content remains easily accessible with the external tools, e.g. lz4cli.