Another utility, for people trying to work around this: https://gist.github.com/Tblue/62ff47bef7f894e92ed5

What I'm wondering now - why compress at all? For me, and I guess for most other users, this file doesn't even reach a Megabyte. Yes, lz4 can compress it to around 10%, but who cares at that size?

It does less I/O when storing and retrieving on disk.

Ok from a quick benchmark, I'd zuggest switching to zstd? It seems to compress/decompress slightly faster than LZ4 with triple the compression ratio...
(I unpacked a ~4MiB upgrade.jsonlz4 I had in my profile and duplicated its contents random times, than ran multitime with mozlz4 (python3 implemented, that may skew the results), zstd and pigz commands packing and unpacking the test file.. on a tmpfs, with BOINC paused on my hexacore machine)

652M test.json
123M test.json.gz
144M test.json.lz4
 43M test.json.zst

Compressing:

++ multitime -v zstd -kf test.json
===> Executing zstd -kf test.json
===> multitime results
1: zstd -kf test.json
            Mean        Std.Dev.    Min         Median      Max
real        1.266       0.000       1.266       1.266       1.266       
user        1.363       0.000       1.363       1.363       1.363       
sys         0.178       0.000       0.178       0.178       0.178       

++ multitime -v sh -c 'mozlz4 -c < '\''test.json'\'' > '\''test.json'\''.lz4'
===> Executing sh -c "mozlz4 -c < 'test.json' > 'test.json'.lz4"
===> multitime results
1: sh -c "mozlz4 -c < 'test.json' > 'test.json'.lz4"
            Mean        Std.Dev.    Min         Median      Max
real        1.353       0.000       1.353       1.353       1.353       
user        0.852       0.000       0.852       0.852       0.852       
sys         0.497       0.000       0.497       0.497       0.497       

++ multitime -v pigz -kf test.json
===> Executing pigz -kf test.json
===> multitime results
1: pigz -kf test.json
            Mean        Std.Dev.    Min         Median      Max
real        2.010       0.000       2.010       2.010       2.010       
user        22.482      0.000       22.482      22.482      22.482      
sys         0.454       0.000       0.454       0.454       0.454       

Decompression:

++ multitime -v zstd -dc test.json.zst
===> Executing zstd -dc test.json.zst
===> multitime results
1: zstd -dc test.json.zst
            Mean        Std.Dev.    Min         Median      Max
real        0.368       0.000       0.368       0.368       0.368       
user        0.363       0.000       0.363       0.363       0.363       
sys         0.004       0.000       0.004       0.004       0.004       

++ multitime -v sh -c 'mozlz4 -d < test.json.lz4 > /dev/null'
===> Executing sh -c "mozlz4 -d < test.json.lz4 > /dev/null"
===> multitime results
1: sh -c "mozlz4 -d < test.json.lz4 > /dev/null"
            Mean        Std.Dev.    Min         Median      Max
real        0.863       0.000       0.863       0.863       0.863       
user        0.408       0.000       0.408       0.408       0.408       
sys         0.455       0.000       0.455       0.455       0.455       

++ multitime -v pigz -dc test.json.gz
===> Executing pigz -dc test.json.gz
===> multitime results
1: pigz -dc test.json.gz
            Mean        Std.Dev.    Min         Median      Max
real        1.467       0.000       1.467       1.467       1.467       
user        2.184       0.000       2.184       2.184       2.184       
sys         0.174       0.000       0.174       0.174       0.174       

For anyone coming across this needing to decompress these files:

/* 
NOTE: BEFORE RUNNING THIS SCRIPT, CHECK THIS SETTING:
Type or paste about:config into the address bar and press Enter
Click the button promising to be careful
In the search box type devt and pause while Firefox filters the list
If devtools.chrome.enabled is false, double-click it to toggle to true

Paste this entire script into the command line at the bottom of the Browser Console (Windows: Ctrl+Shift+j)
Then press Enter to run the script. A file picker should promptly open.
*/

async function convert() {
  // Set up file chooser
  var fp = Components.classes["@mozilla.org/filepicker;1"]
    .createInstance(Components.interfaces.nsIFilePicker);
  fp.init(window, "Open File", Components.interfaces.nsIFilePicker.modeOpen);
  fp.appendFilter("Bookmark Backup Files", "*.jsonlz4");
  var result = await new Promise(resolve => fp.open(resolve));
  // Call file choose, proceed if a file was chosen
  if (result == Components.interfaces.nsIFilePicker.returnOK) {   var file = fp.file;
    // Check that file can be used
    if (file.exists() && file.isFile() && file.isReadable()) {
      var oldfile = fp.file.path;
      // Construct output file name
      var newfile = oldfile.replace(".jsonlz4", "_converted.json");
      // See: http://forums.mozillazine.org/viewtopic.php?p=14111285#p14111285
      var {utils:Cu} = Components;
      Cu.import("resource://gre/modules/osfile.jsm");
      var jsonString = await OS.File.read(oldfile,{ compression: "lz4" });
      // console.log(jsonString);
      OS.File.writeAtomic(newfile, jsonString);
    }
  }
}
convert()

The severity field for this bug is relatively low, S3. However, the bug has 25 votes and 52 CCs.
:mossop, could you consider increasing the bug severity?

For more information, please visit auto_nag documentation.

The last needinfo from me was triggered in error by recent activity on the bug. I'm clearing the needinfo since this is a very old bug and I don't know if it's still relevant.

I don't think there is any value in posting further comments in support of this, we're on board with doing it, we just don't currently have the resources available to do it. If someone wants to work on implementing it then we would review a patch.