Your block list is stored in a SQLite database. Here's how you can extract it.

(1) Open an online SQLite database viewer such as: https://inloop.github.io/sqlite-viewer/

Keep this tab open because we are going to drag and drop a file onto it

(2) Open your currently active Firefox profile folder. This article has the steps: https://support.mozilla.org/kb/profiles-where-firefox-stores-user-data

Size this window so you can see the SQLite site behind it

(3) Double-click into the gm_scripts folder, then scroll down to the Google_Hit_Hider_by_Domain_(Search_Filter_Block_Sites).db file (sometimes the .db extension is not visible, but it will have a different icon from the folder of a similar name)

(4) Drag the database file onto the SQLite viewer page and it should immediately extract five data fields. The one you want is hideyhosts -- you can paste it into the Import panel of GHHbD running in Tampermonkey or Violentmonkey.

Hmm, first post isn't editable...

For step (1) I also created an extractor here: https://www.jeffersonscher.com/gm/gmextract.html