Tested with:

Nightly 88.0a1 (2021-02-25)

Tested on:

Windows 10
MacOS 10.15

Preconditions:

In about:config, set browser.proton.appmenu.enabled = true

Steps:

1. Launch firefox and sign in into FxA using 2 devices.
2. Open more than 25 tabs on one device and sync tabs with another devices.
3. Click on the "Show More" option from the Toolbar FxA menu.

Actual result:

If the user expands the list of synced tabs from the Toolbar FxA menu, it becomes wider.

Expected result:

The toolbar FxA menu should not change its size.

Notes:

1. The issue does not occurs in the Hamburger Menu.
2. The issue is reproducible both with and without Proton enabled.