Serpent Tester Tool is designed for testing compatibility of Pale Moon specific add-ons with Basilisk. It allows to install extensions with only Pale Moon in manifest. The extensions installed via Serpent Tester Tool are marked with [TEST] (or [FIX] for themes) at the beginning of the name and can't be updated automatically.
Installation of improper extensions or themes may break the browser and lead to the damage or loss of data. You can't expect or ask for any support of extensions and themes installed via Serpent Tester Tool neither from its authors, nor from developers of the browser.
If you find the extension or theme that runs well, you have to contact author and ask him provide compatibility with Basilisk officially.