In the past few days, Sync stopped syncing all my tabs. Previously if I used about:sync-tabs I could see all the tabs I had open on every machine. I could double-tap one to open it locally, at which point it would disappear from the list of remote tabs. Now, only a tiny subset of the tabs from each machine appears in the sync-tabs list. My main desktop has something like 30 tabs open yet only around 8 appear in the sync-tabs list on my laptop. I've tried restarting each FF instance along with forcing a manual sync to no avail. If I open new tabs right now they *do* appear in the sync-tabs list, and if I close one of the tabs from the list it goes away - so tab syncing is at least partially functional.

The easiest way for me to repro this is to restart FF on my desktop then Sync. Most of my tabs disappear from the sync-tabs list. If I manually select the tabs on the desktop so they load, then force a Sync again, that usually makes them reappear on the list. So I think this might be as simple as tabs disappearing from sync until they've been manually loaded by selecting them? Pinned tabs don't sync either, but that's a known caveat, I think?

I don't have a reliable test case for this, but it's consistently broken. My home desktop typically has upwards of 50 tabs open, with sync enabled and working...

And here's the synced tabs page from my windows tablet after I forced a sync. The most recently opened/visited tabs are there, but nothing else.

Thanks for the explanation. We do limit the number of tabs to 50, and bug 1244622 exists to offer some way to see a larger amount - hopefully we will get some traction on that soon.