Sync is very useful and should become even better when the new Profile Management System lands.
However, there's still a few things that could be worked on. Real-life example: I have 2 PCs, one I use daily and one I use once in a while. Sync normally allows me to keep my bookmarks (among other stuff) up-to-date across both. But, if I log in to the first and apply changes to my bookmarks (say I delete some bookmarks), when I log in on the second a few days latter, sync merges the bookmarks across the two devices bringing back the bookmarks I had deleted. Something similar happens with Homepage.
The way I see it, Sync needs to be more like git and could use at least a few options:
One session / machine should be declared as main,
Other sessions / machines are automatically branches,
When logging in to an account, Firefox should ask whether to "keep the main", "update main based on current branch" or "merge the two" (like what it's doing currently),
Doing so should display a summary of changes; this could be like git-diff per synced item (AI-generated summaries is another plausible venue nowadays I guess). Importantly, the summary should highlight which changes come from which session / machine and what are their timestamps,
Possibly add a confirmation dialogue as a final step to avoid accidents.
Functionality (3-5) could be exposed to the user as an option per-profile with some default parameters.
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