For unknown reason http://www.opensubtitles.org
was added to some list of domains where FireFox is blocking downloads.
In meantime we contact developers and remove our site from list, you can do following:
How to fix zero size downloads in FireFox on Windows:
1) Click DOWNLOAD (beta) on website
1) once you clicked on DOWNLOAD button on website, open in FF downloads (CTRL+J)
2) you have to click on "Open"
3) file will be in your default download folder
1) if there is Download (beta) button on
1) use other browser, Chrome, Opera or Edge should work normally
or (not recommended)
1) FireFox: Menu -> Options -> Privacy & Security
2) Unclick checkbox in Block dangerous downloads
More reading: https://support.mozilla.org/en-US/kb/ho ... is-blocked