Top Sites extension inconsistently resizes after first open

Steps to reproduce:

1. Install the "Top Sites Button" extension.
2. Open it for the first time in a new window
3. Close the extension window and open it again

Expected behavior:

The expected behaviour from what I know would be for the size of the extension popover window to stay consistent.

Orion, OS version; hardware type:

MacOS 12.0.1, iMac 2020

Image/Video:

First Open in new window:

What it changes to:

Firefox screenshot (Size never changes):