https://sites.google.com/view/highlight-mxm/changelogs/firefox-changelogs/231Firefox-M1x8R5v3

Your setup follows your Firefox account:
Lists, folders, word rules, combination rules, pattern rules, theme presets and settings are stored in your Firefox account, so signing in on another computer brings them with you. It is on by default and needs no setup beyond being signed in to Firefox. The record of when each word was last seen — what the inactivity report reads — stays on the device, since it describes what this browser has encountered rather than how Highlight is set up.

A switch, in Settings → Backup:
Sync across my devices turns it off and back on whenever you like. Turning it off moves everything back onto this device and clears what was stored in your account; turning it on puts it back. Nothing is lost either way, and the line underneath always says where things currently stand.

Large lists stay put instead of failing:
Firefox caps a single synced item at about 8KB and the whole account at about 100KB — a list of 400-500 words reaches the first cap on its own. Rather than let a save fail, anything that doesn't fit is kept on this device and marked local only on its row. The rest keeps syncing. Export/Import still moves the oversized ones by hand.

The total limit is handled too:
Items that each fit can still exceed the account's overall ceiling together, and no size check can see that coming. When Firefox refuses a write, that item falls back to this device rather than being dropped — so a large collection degrades one list at a time instead of breaking.