Rated 5 out of 5 stars

I really liked this great addon and got quite disappointed with this previously unknown 0kb bug.

After some research, I figured out that actually it was caused only by mp3 files not being converted correctly after the download.
As I can see that I'm definitely not alone facing this issue, I decided to share this fix which definitely worked for me :-)

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So, to begin with, first of all I would like to underline that disabling "verified addons" feature is NOT necessary, use it at your own risk along with any other actions described below.
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Ok, now, basically all of the following steps are needed only to replace ffmpeg.exe after installation, so that Firefox wouldn't disable this addon later after browser's restart as it will become "unverified" after the fix. Unfortunately, I couldn't find the way to disable the check only for one specific addon.

To prevent it, this feature can be deactivated:
https://support.mozilla.org/en-US/questions/1078339

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste xpi and pause while the list is filtered

(3) If the xpinstall.signatures.required preference is bolded and user set to true, double-click it to switch it to false

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2. Get User Agent Overrider
https://addons.mozilla.org/en-US/firefox/addon/user-agent-overrider/

3. Spoof the browser with Windows/Firefox 29 - this will allow to install the previous version of youtube-mp3-podcaster (I used 3.8.0b)

4. Go to development channel,install 3.8.0b and RESTART the browser:
https://addons.mozilla.org/en-US/firefox/addon/youtube-mp3-podcaster/versions/beta

5. Download the latest stable version of ffmpeg (currently 3.3.3) here:
http://ffmpeg.zeranoe.com/builds/

6. Open your FF profile:
%APPDATA%\Mozilla\Firefox\Profiles\

7. Go to
\extensions\youtubemp3podcaster@jeremy.d.gregorio.com\chrome

8. Replace the file ffmpeg.exe in this folder with the one from the downloaded archive ffmpeg-3.3.3

9. For RU (and I suppose FR/DE/ES/anyone else who use special symbols) users - Unicode path fix should be ENABLED in the addon's options.

10.Youtube-mp3-podcaster will start showing command conversion screen after downloading the file - just wait for it to finish the conversion.

That's all, everything should be working now! :-)

And yes, like everyone else I'm looking forward for official "verified" fix, many thanks in advance!