QuickSearch (PotatoSearch™)

From https://marketplace.firefox.com/search/ you can include any of the following search keywords. You can also compound keywords (e.g., :free :packaged maps to show free packaged apps related to "maps").

Reference

:packaged

Return all packaged apps

:hosted

Return all hosted apps

:free

Return all free apps

:free-inapp

Return all free apps with in-app purchases

:premium
:paid

Return all paid apps

:premium-inapp
:paid-inapp

Return all paid apps with in-app purchases

:premium-other
:paid-other

Return all paid apps using an other payment system (i.e., not via the Marketplace)

:cat-<category_slug>

Return all games (e.g., :cat-games for Games)