How to convert old sessions from "sessionstore.js" to the newer "sessionstore.jsonlz4" format files
Story Time aka Scrambled mess
So, for the Longest time I've had a Bunch of old sessionstore.js backed up and since I had to reset my Windows a couple of days ago, I thought about going through them and deleting what I don't need/saving what I do need. Problem is that the most recent version of FF use sessionstore.jsonlz4. I tried as hard as I could to make the .js work, but nothing work. I tried to convert the .js to .jsonlz4, but of course that did not work either. The only thing useful I found was this Scrounger that finds the websites within the .js so you can manually save them... But that's not good enough for me. So I thought "Wait a minute... .js was used by old version of FF, then FF updated and started using .jsonlz4... but what about... everyone that was using the old version when it converted ? Wouldn't FF itself convert the .js to .jsonlz4 automatically if you upgrade from an older version ???" So I download a portable version of FF 52, open it up for the first time, set it to remember my last session, close it, copy and old sessionstore.js from my backups and paste it into the portable's profile folder and VOILA it magically worked !!! This was the first Win I've had IN A WHILE so I really needed it. Now I am still in the process of doing it but theoretically you can copy the profile from the portable to your main installation of FF. Oh and as a bonus, you can go to about:profiles, create a new profile and replace its contents with the portable one's.
- Preferably download an old Portable version of Firefox (I used this site and got Version 52 but you are free to use whatever you want).
- Launch the portable version and navigate to the settings to select it to Remember your last session.
- Find the Portable's Profiles folder, not your main installation of FF.
- While FF is closed, Paste your old sessionstore.js file into the Portable's Profiles folder, overwriting the existing one it had. (theoretically you can also restore your sessions with your old previous.js and recovery.js/bak files by renaming them to sessionstore.js and putting them in Portable's Profiles folder.
- Open Portable FF and you Should see your old session. Now open the Menu Bar with Alt -> Help -> About Firefox -> Update Firefox -> Restart Firefox.
- I suggest Updating Portable FF multiple times until you reach the current version, but FF 55 or something should be enough but hey, there's nothing wrong with being safe.
- At this point you should have a FF profile with your old session in the new .jsonlz4 format. But why stop there ? Open your regular version of FF that you have installed (aka Not the Portable one) and type about:profiles in the URL bar.
- Select Create New Profile. Now you have 2 options: A. Copy the Profile folder from the Portable FF into normal FF's Folder for the profiles, or B. just select Finish and paste the contents of the Portable Profile's folder in the newly created profile in the normal FF. Oh and you can rename the profile too to something like "old sessions"
- After doing that, in about:profiles it will change the Default profile to the newly created, I suggest changing it back to the one you were using. Click Launch Profile in new browser.
- From there you can decide how you want to save that session. You can sign in to your FF account and sync it with your main and those tabs will be in "Synced Tabs".
.....Sorry for the mess, I am not good at explaining things and I try to be thorough.
Improve my solution
I guess you can improve whole thing by finding exactly the last version that used sessionstore.js so you only have to update Once to get the sessionstore.jsonlz4.