If that extension/add-on is the one that is messing up your Firefox profile, you need to uninstall it. It may not be compatible with the version of FF you are using.
Then, you might try a different downloader. There are many out there — some as add-ons and some as standalones.
I’ve tried different downloaders, and none of them worked. It wasn’t until I created a new profile that the problem disappeared, for all of about 3-4 days, then it came back.