Firefox lets you browse or search through cache entries on this page: about:cache (type or paste that into the address bar and press Enter). The detail will show you where the file is stored on disk, if it is stored on disk.

However, be forewarned that if you accumulate a lot of files in the cache, it's quite slow, and many videos are never cached, so if you regularly need to save videos, you may need to use an add-on.

Note that it is more likely that Firefox uses the memory cache for videos and doesn't store the video to disk. Some (streamed) videos can be downloaded in chunks and only the current chunk is in the cache and not the full video.