Limitation of the Firefox Extension to upload a group of file

When I dropped a directory containing 4 files, the import worked fine, but when I tried a slightly bigger directory (like 720 files), then I regularly got a firefox alert saying that the page contain an unresponsive script ("A Script on this page may be busy, or it may have stopped responding...").

When I asked to continue, the download did continue, but the question was keeping popping up to the point where I had to stop the upload.