User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:87.0) Gecko/20100101 Firefox/87.0

Steps to reproduce:

1. Have a Firefox account from some installation with addons and changed settings/opened tabs... etc.
2. Clear install Firefox for example in a different OS account or on a different PC
3. Login to the Firefox account in that second installation

Actual results:

Firefox starts sync of everything, history, opened tabs, settings/preferences, highlights and addons.

Expected results:

Firefox should have asked on each new installation what should be synced exactly from and to this specific instance of a Firefox installation to/from the account.

For example:
I login to this new installation and don't want some addons or the preferences to be synced from the account and/or to the account, i just want the share tabs functionality between the different instances.

I should have been asked what exactly i want to sync before the initial sync actually begins. I believe Chrome acts this way, but i am not sure.

Nowadays when this feature is not implemented if i remove some addons the new installation they are also removed from the other accounts, and as a new user i do not expect this to happen and only notice later. As a new user i also probably don't remember what addons exactly were removed to restore them.

Another scenario - i don't want the preferences from the Firefox account to be synced at this new installation, now i have to restore them, but as a new user i probably don't know that this will also make my old/regular installation change it's settings to the new installation.

Another scenario - i don't want my bookmarks to be available in this new installation. So when i see them in my new installation, as a new user i just decide to go the easy way without thinking it up and simply delete the bookmarks from the new installation. Unfortunately this will also delete them from my main installation without my knowledge and without a way for me to restore them.

The Bugbug bot thinks this bug should belong to the 'Firefox::Sync' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Sync options are per-account and not per-device. Doing things per-device is trickier than it sounds with its own set of edge-cases and complications, but we are tracking that over in bug 1215415, so I'm going to close this as a dupe of that.