Videos can no longer be downloaded

I have been using Firefox (for Linux Desktop) to download videos for offline viewing for a long time now. I've used DownloadHelper and Flash Video Downloader addons and they've always done the job, just fine. However, after updating to Firefox 29.0 yesterday or the day before, they will no longer download videos. I've tried a few other addons (to see if maybe it was the addons themselves), but the result is the same -- I've tried it on different websites, as well, but it still doesn't work. The download will initiate, 1-6Mb will be downloaded, then it will "fail." Even with a download manager that allows you to resume paused or broken downloads, it will fail. The strange thing is, if I download a video without using any addon, it works. The issue is, in many cases it's not possible to download a video without the help of an addon. I think maybe there's something wrong with the way Firefox is handling the downloading of videos through addons. Is there a fix for this? I'm happy to provide technical information (browser configuration, etc.) if it'll help.