Unless you are using external download manager like IDM or FDM you won’t see the file size when you try to download a file from a page, or by default Firefox won’t shows it. Its always better to know size of file we’re downloading, so we’re always can decide to download that file or not depending on file size and our internet connection speed.

show file size shows file size in Open with dialog box

Show File Size Firefox add-on, shows file size in the Open With dialog box.

Show File size Firefox add-on