[hacliff]

Depending on how they're writing their files; firefox doesn't support persistent file storage last time I checked.

Firefox does have indexeddb, but if you want to do seek operations you're out of luck.

EDIT: To explain more, without seek writing dealing with large files is incredibly expensive, you have to write everything you've got to indexeddb every time you want to modify it.

[doublec]

The FileHandle API does what you want - read/write access to an IndexedDB file at any location. https://developer.mozilla.org/en-US/docs/WebAPI/FileHandle