How to force Firefox to open a file which does not have a .txt extension as if it did have one?

For a quick solution, you can add view-source: before the url. This also works in Chrome.

You should try this addon: Open in Browser.

It offers the possibility to display documents in the browser window.

for nightly 26 alpha, opening .java files as text/plain

find ~/.mozilla -name mimeTypes.rdf

add children to node /RDF:RDF

<RDF:Description RDF:about="urn:mimetype:text/plain"
               NC:value="text/plain"
               NC:editable="true"
               NC:fileExtensions="java"
               NC:description="Java source file">
  <NC:handlerProp RDF:resource="urn:mimetype:handler:text/plain"/>
</RDF:Description>