For some downloads, Firefox reports the file size as -1 which may be wrong. In such cases, uGet Integrator will not capture the download. To interrupt all downloads, please set the minimum file size to -1. Otherwise, use the Always interrupt keywords to interrupt your desired files.

If the above solution does not work, please ensure that the architecture(32 bit or 64bit) of Firefox match with your operating system.

If that does not help, delete handlers.json from the following path:

Firefox Stable:

~/.mozilla/firefox/mwad0hks.default/handlers.json

Firefox Nightly:

~/.mozilla/firefox-trunk/mwad0hks.default/handlers.json

Where mwad0hks is a random string so you may have a different folder name.

If the above solution works but does not persist, the solution provided in #3 may give you a permanent solution.

For more details, please check issues #43 and #3.