Extension icon pinned to toolbar is invisible

Description

I've installed an extension (Checker Plus for Google Calendar™ by Jason Savard). It installas fine, and it works fine. But the extension icon is only accessible in the Extensions dropdown menu After I rightclick the extension and choose "Pin to Toolbar", the icon disappears from the dropdown but is not added to toolbar - it disapears completely. The only way to get it back is to "Restore Defaults" in "Customize Toolbar" editor.

Steps to Reproduce

Install "Checker Plus for Google Calendar™ by Jason Savard"
See the extension is available in "Extensions" dropdown
Rightclick the extension and "Pin to Toolbar"
Notice the extension is gone, there is no icon on the toolbar nor in the dropdown.

Additional Information

n/a

Affected Websites

This also happened with some other extensions, e.g. "Simple Translate" by sienori

Environment

Firefox 131.0.3 64bit firefoxpwa 2.15.5

filips123 / PWAsForFirefox