At any point in time, I have many Firefox tabs opened. I might even have reached a thousand. This feels like an issue I should address at some point, and I have already tried. But today, my goal is easier : count the number of currently opened tabs. In truth, it’s fairly easy : many extensions provide this information, in more or less granular details (number of windows, number of tabs in a specific window…). However neat this is, I want to get my total number of opened tabs printed in a Unix shell. Indeed, I ultimately would like to automatically graph this number over time.

Before I started, I figured I’d need to find a session file, list the tabs, and get the count. Sounds fairly easy right ? Well it was not too hard in the end, but there were a couple bumps. The first is the format used to store the sessions. It’s compressed using LZ4, but with headers set by Mozilla, before LZ4 was fully standardized. This StackOverflow answer gives a bunch of tools to decompress the file. I’ve chosen to use andikleen’s tool as it worked out of the box easily. Once in possession of the extracted JSON, I’ve used jq to get the tabs’ IDs, and wc to count them.

So with ./lz4jsoncat /home/<user>/snap/firefox/common/.mozilla/firefox/<profile-id>.default-release/sessionstore-backups/recovery.jsonlz4 | jq '.windows[].tabs[].index' | wc -l, I’ve gotten my answer in a shell : 138 (yikes !). I’m using only one Firefox profile, but I suppose you could adapt this shell pipe to support more. Anyway, I’ll need to work on a cronjob to do it regularly, and an agent to collect these data (probably Prometheus). This would be one of the first metrics I’d collect for my "local computer monitoring center".