My indexedDB folder contains folders corresponding to two different websites, each filled with a plenitude of contents. My guess is that were I to delete the files and revisit those sites, having found their data missing, scripts would promptly replenish their contents. However, that's just supposition and it presumably depends on what exactly those sites stored in the folder. Certainly if it's content you paid for, you might prefer not to trash it.

This folder is used for use by applications to store data for offline usage.

• Firefox/Tools > Options > Advanced > Network > Offline Web Content and User Data

You would normally have to give Firefox permission to store data this way, so you should know where it comes from.

You can inspect and manage the permissions for all domains on the about:permissions page or for the current domain via the "Site Identity Button" (globe/padlock) on the location bar and click "More Information" to open "Page Info" and go to the Permissions tab.

• https://support.mozilla.org/kb/how-do-i-manage-website-permissions