Sign in to Firefox Sync, but don't sync bookmarks?

I did some research online and some experimenting in a virtual machine, and the only way I found to achieve what you want is as follows:

1. Make sure you have the "Preferences" button (or "Options", depending on your OS) pinned in a convenient spot on your toolbar. This is very important, as it helps you to complete the following steps more quickly. If you don't already have it on your toolbar, then add it by clicking on the button with 3 horizontal lines (located on the toolbar at the far right), then selecting "Customize" from the menu, and dragging "Preferences" (or "Options") to the toolbar in an easy to reach spot. Then click "Done" to save your configuration.

2. Log in to your Firefox account; if you use 2-factor authentication, complete that step before proceeding.

3. Immediately after completing the login process, click the Preferences/Options button on your toolbar, and on the Preferences page, click "Sync" (or "Firefox Account", depending on your version of Firefox). Then in the box that says, "You are currently syncing the following items", click the "Change" button. In the dialog that pops up, uncheck "Bookmarks" and click "Save Changes". You must do all this very quickly, before the syncing process has time to complete.

I hope this helps you!

Kind regards,

computerworm10100