VLC converting files on Download

I searched and didn't find anything on my problem here or anywhere. I am hoping the answers are here. I am using Download helper in firefox for Linux, I am running MX Linus 19.2. Download helper has the option to convert files on the download. The question I have is there are very many choices to convert to like 10 choices for MP4. I don't know which one to chose as most of them are unknown to me. I am trying to download mkv files and convert them to MP4. It is much faster tan trying to use a converter to change formats. I hope someone is familiar with these and can tell me which one to use. Thnak you in advance for your time and advice.
Pablo