User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

• Change default Fonts
• (Preferences > General > Language & Appearance)
• Delete Firefox profile from ~/Library/Firefox (macOS).
• Login again with same Firefox account.

Actual results:

It didn't retain Fonts changes.

Expected results:

Should save and keep all preferences in sync.

Tested this issue on Mac OS X and Windows 10 as well - on several Firefox versions: latest nightly 67.0a1, beta 66.0b10 and release 65.0.1.
The issue can be reproduced on both machines Windows, MAC OS X
I will add a new description in order to reproduce this issue:

[precondition]: You must have a Firefox account.

1. Open one of the above specified Firefox version.
2. Go to hamburger menu of Firefox and click on the 1st option "Sign in to Sync"
3. Enter your Firefox credentials account.
4. Then, go to -> Preferences/Options -> Language & Appearance and change the font size.
5. Close Firefox.
6. On Mac OS X, go to ~/Library/Application Support/ Firefox/Profiles and delete your profile (created earlier).
7. Open Firefox with a new profile and from Hamburger menu-> "Sign in to Sync" -> access the Firefox account for syncing data.
8. Inspect the font size from Preferences/Options -> Language & Appearance or do a random search on browser.

Hmm, we could certainly sync these prefs. Some things to keep in mind:

• What if you're syncing between two Firefoxes with different locales? We can likely handle this by syncing all font-related prefs...but there's a lot of them!
• What if your systems don't have the same fonts? I think we'll still do the right thing, since we always need to fall back to something, anyway, but it's worth double-checking.

Marking this as an enhancement, as we've never supported syncing them. Thanks for filing!

(In reply to Lina Butler [:lina] (ex-Mozilla) from comment #2)

• What if your systems don't have the same fonts? I think we'll still do the right thing, since we always need to fall back to something, anyway, but it's worth double-checking.

I have a suggestion for the second question:

Allow the user to do a font stack, by selecting two or more fonts to each selection. Of course, the drop-down menu would need to be substituted by pop-up window with a font selection list panel on one side (populated with all fonts on the system) and a second panel populated with the user selected fonts that can have its order arranged.

Or, an easier way, just a pop-up window similar to the font selection dialog found within LibreOffice Writer, that allows the user to write the font name or a semi-colon separated list of fonts, so the power user can write its font stack there. LibreOffice Writer accepts that so well that I use those "portable font stacks" that travel with the documents I created, I think Firefox could handle that just as fine (as it already does with the web pages font stacks).

I will attach an example.

Allow the user to do a font stack...
I think Firefox could handle that just as fine

It have been possible for many years. Just use font.name-list.* instead of font.name.* preferences. E. g.:

user_pref("font.name-list.monospace.x-western", "Iosevka SS16 Light, PT Mono, monospace");

instead of

user_pref("font.name.monospace.x-western", "Iosevka SS16 Light");

Dunno, why about:preferences confines users to the latter.

In any case, even wishlist issues are better been reported separately, not in (un)related threads.