1. Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. ...
2. Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.
'No video with supported format and MIME type found' issue support.mozilla.org > en-US > questions
Fix Video format or MIME type is not supported error in Firefox
1. Open the Firefox browser and click on the hamburger menu button in the top right side corner.
2. Then click on the options and select the privacy and security in left side menu.
3. Scroll down to the Cookies & Site Data section and click on the Clear Data button.
Video Format or MIME type is not supported error in Firefox [Fix] howtodoninja.com > how-to > video-format-or-mime-type-is-not-supporte...
MP4 Videos Require a MIME Type on Your Server to Play Properly
1. Right-click the site in IIS and select Properties.
2. Select the HTTP Headers Tab.
3. Select File Types.
4. Under the MIME Map section, select New Type.
5. Type ". mp4" as the associated extension and "video/mpeg" as the content type.
6. Click OK.
MP4 Videos Require a MIME Type on Your Server to Play Properly articulate.com > support > article > mp4-videos-require-a-mime-type-on-yo...
Click the icon in the top right corner and select "MIME Type" to display them.) Select a file type entry and click 'Change Action...' If you want files of that type to open with the default application, select "Open them with the default application".
File types and download actions - MozillaZine Knowledge Base
kb.mozillazine.org > File_types_and_download_actions