That's hard to fix because in Firefox 57+, extensions cannot directly modify how the Download dialog works.

There are extensions which will modify the content-disposition header associated with the file to tell Firefox it's not forced to handle it as a download. You probably don't want to keep this one turned on all the time because it doesn't let you pick and choose the file extensions it should be applied to. But you could see whether it works on the .collab files:

https://addons.mozilla.org/firefox/addon/inlinedisposition-reloaded/

This seems to work with the .collab files. A couple followup questions then...

1. What is the potential downside to leaving this on all the time?
2. [more importantly] Why doesn't the default "Do this automatically..." just work?

Thanks.

gtilflm said

1. What is the potential downside to leaving this on all the time?

Sometimes you may want to download a type of content that Firefox can display in a tab, and this extension would override the headers set by the site. For example, it could affect text, html, css, javascript, jpg, mp3, etc. But if you right-click > Save Link As on the link, that should still work.

2. [more importantly] Why doesn't the default "Do this automatically..." just work?

According to the author of the extension you linked to, Firefox ignores "Do this automatically" when a file is preceded by a header with content-disposition:attachment (forced download) instead of content-disposition: inline. That's why I suggested the new extension as a workaround, it switches the header.

Thanks for the info. Any chance that this addon (https://addons.mozilla.org/en-US/firefox/addon/do-this-automatically/) or something like it will get re-included soon? That one was brilliant because you could specify by application.

I don't know about other add-ons. I can only see what you can see...