There are a couple of limitations at play here.

Firefox Sync will not send all history because it would simply be way too much data to transfer. Only history items that are less than 30 days old are sent to the Firefox Sync servers for storage. When history items are older than 60 days, they are removed from the Firefox Sync servers and are no longer sent to other devices. Additionally, Firefox Sync can only store up to 5000 history items.

However, the Firefox Sync limitation doesn't apply to data that's already on your browser.

That said, the browser itself also has a limitation to how many history items it will save. As you can imagine, there are some users that have been using Firefox for the past 20 years. Trying to save all of that browsing history would take up quite a lot of space and trying to load that many items into the history window would crash the browser because it's too much data.

The limit for how much history is stored in Firefox is determined by the number of items, not by time. That limit is customized automatically based on the specific specs of your computer. You can see what that limit is by looking at the places.history.expiration.transient_current_max_pages preference on the about:config page.

NOTE: That preference can be edited, but it doesn't change how much history is saved because it gets reset each time you start Firefox.

Hope this answers your question.

Thanks for your anwser, Wesley!

Since it's a limitation by the server: Could I change the server-side limit of synced items if I'd run my own FF Sync server?

If not, can I somehow merge histories of different profiles after changing `places.history.expiration.transient_current_max_pages` or `places.history.expiration.max_pages` to a larger value? I also have profile backups with a much older history in them.