Local .md file rendered as plain text on Linux

Ubuntu 20.04, Firefox 100.0 installed manually to `/opt/` from web download, Markdown Viewer Webext 1.8.1

When I try to load a local .md file, the file loads and is displayed (no download dialog), but renders as plain text, and isn't formatted. Same result with any local .md file.

I've followed the [instructions to configure the MIME type](https://github.com/KeithLRobertson/markdown-viewer#support-for-local-files-on-linux), except I used `/usr/local/share/` instead of `~/.local/share/`. This is my MIME file:
```

```

IIRC, it worked correctly on an earlier version of FF, back when I set it up. I haven't used it in a while, but now it's not working, as described.

I have uBlock Origin, but disabling it makes no difference. Disabling all other extensions also doesn't help.

Are there some FF settings that may be blocking the extension scripts? What can I do to debug this further?

Start by reproducing the local .md loading behavior with Ubuntu 20.04, Firefox 100.0, and Markdown Viewer Webext 1.8.1, then review the repository's local-file MIME configuration instructions and the reported MIME file. Done means local Markdown files render formatted rather than as plain text; compare behavior with extensions disabled as reported.

Written by the indexing model from the issue text.