Affected Version:
Latest Nightly 88.0a1 (2021-03-12)
browser.proton.enabled

Tested Platforms:
Windows 7/10, MacOs

Steps to reproduce:

1. Go to any site and bookmark it
2. Notice the "Saved to Library!" toast dropping down from the hamburger menu button
3. Open the menu

Expected Result:
The user expects a section named "Library" which was suggested by the displayed toast.

Actual Result:
Due to Proton changes the library section is no longer available in the menu.
Screenshot: https://drive.google.com/file/d/1Up_dLAF8bwxSD8lW-_0fs3K_4XMBSM2h/view

For users without the library icon in toolbar, the toast message should be updated to: Saved to bookmarks

Romain, for users that will still have the library icon in the toolbar, I assume toast will continue to drop from there? And in that case the string would be: Saved to library

Users that have the toolbar library icon have a different animation (star dropped into the toolbar icon without the toast).
So I think for the purpose of this bug we should limit scope to adjusting the string from "Saved to Library!" to "Saved to bookmarks"

Marking as P1. Per experience review we agreed to mark as P1 bug the ones that will block MR1.

Not porting to Fluent here because updating ConfirmationHint to use Fluent
strings for all hints is out of scope.

https://hg.mozilla.org/mozilla-central/rev/ee6e2d058168

Verified-fixed on the latest Nightly 89.0a1 (2021-03-25) on Windows 7/10, MacOS 10.15 and Ubuntu 16.04.