Not without 'rooting' the phone. The android system aggressively sandboxes each app into its own, isolated, part of the filesystem. So there is no 'official'

Not without 'rooting' the phone. The android system aggressively sandboxes each app into its own, isolated, part of the filesystem. So there is no 'official' way to see the app local files, other than by the app itself showing them to you unless you can root the phone. But, you can go into the settings -> apps section, tap on the storage used area, and press "clear cache" to clear some of the data, and in Firefox go into settings -> "clear private data" and then choose what to clear and delete that and likely recover a fair amount of space.