The normal LZ4 header is only 11 bytes, so we need to strip that of first, this can either be done with tail -c+12 4 > or with dd bs=11 skip=11 if=4 of=. Generating this header is easy enough, a simple cat fixedsession.json | wc -c gives us the amount of bytes in the actual file, and a simple echo -n along with some hex escape codes can be used to output text to a file without a newline. This is followed by a 4 byte 32-bit integer (probably unsigned) recording the original file size in little-endian. In this case that 8 byte magic is simply the NULL terminated string "mozLz40". Looking around the web the mozlz4 format is said to have an 8 byte magic sequence at the start, this is a common thing in file formats that allows utilities to recognise the file even if the file extension is unknown or missing. And even looking at what Xarchiver was running to do the decompression was simply a re-structured version with the standard LZ4 decompression tool. This fortunately means that it is fairly trivial to convert between the two once you know how. Looking for ways to create these files it seems like the format was actually created before LZ4 had a standard on-disk format, so Mozilla created their own. But looking at the output of file show that this is "Mozilla lz4 compressed data". Looking at the extension it looks like it would be a normal LZ4 compressed file.
#Recover sessionrestore jsonlz4 windows
Easy enough right? Just move my window from the _closedWindows over to the windows array, right? Well, that would fix the JSON, but we still need the original compressed file format. And all my nice pinned tabs where in the latter. In this file there is a windows array along with an array named _closedWindows. As a side note the great jq utility that can, amongst other things, pretty print JSON, came in handy here. Looking into the JSON file it was easy to see I'd come to the right place, there where all my pinned tabs, even with history! But the problem seemed to be that I had closed the window which shows the pinned tabs and later closed the browser itself.
So I opened it up in Xarchiver which was happy to extract a nice JSON payload for me. Having a look at the extension of this file quickly made me realise that this was some kind of compressed JSON file. So it was time to dig deeper! The mozlz4 file format Unfortunately after shutting down the browser, replacing the file, and restarting it yielded exactly the same results. Quickly copying this to a safe location in order to not have Firefox rewrite it I was told that simply replacing ~/.mozilla/firefox/fault/sessionstore.jsonlz4 with this file would reset my session. Asking around on the #firefox IRC channel I was quickly pointed to ~/.mozilla/firefox/fault/sessionstore-backups which contains the previous.jsonlz4 file that holds the previous browser state. So you might imagine my dismay when I opened my browser this morning, just to discover that all my pinned tabs where gone! Of course since I only have about 6-8 of them it wouldn't be much work restoring them by hand, but where's the fun in that! And if this was a repeat issue I certainly wouldn't like to have to do it every time. Exactly what I need! Where things went wrong These tabs survives a browser restart, even when you don't have session recovery turned on, and unlike just leftover tabs they don't require you to click on them when you first open your browser to load them. Firefox luckily has a pinned tab feature for this, where you can pin a tab in a window and it will load all the pinned tabs when you open the browser. For example my webmails and various chat applications (both for work and for leisure), my HomeAssistant dashboard to be able to easily control various things around the house, and a couple of those aforementioned articles I just wanted to save for later. As you're probably aware there exists a whole slew of web-apps nowadays, some of which I'd like to have open with my browser. My new system of starting with a fresh session every time would however be a bit tedious if it closed absolutely all my tabs. Part of the problem here is that I've just never been much of a bookmarks person, so I've just kept all the stuff I might want to read one day open as a tab.
My habit was so bad that I could easily amass some 100-200 odd tabs before I cleared them out. This means that every time I turn my computer off for the day it will delete all the random documenation pages and weird links people have sent me over the course of the day so they don't slowly add up.
#Recover sessionrestore jsonlz4 install
In order to stifle my tab-hoarding habits I have recently set my Firefox install to not recover sessions when I restart the browser. Fixing a broken Firefox session 27th April 2021 - Guide
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