drench For what it's worth, it looks like the Firefox extension from this page on their website works with Orion (Update: Firefox version of the extension is depreciated and no longer maintained.)
Context for those that may not know what mymind is: mymind is a visual bookmarking service ala Raindrop.io, not dissimilar to Pocket or Omnivore. Extension's purpose is to capture images, urls, text and send to the service.
A caveat/issue with the mymind Firefox extension that shows up in Orion uniquely is that the "Save to mymind" option does not consistently appear in the right-click context menu, as it does in other browsers (expected behaviour is for it to appear 100% of the time - so it is able to capture the URL of the current page, instead of only appearing when right-clicking text or an image, which is the case when using it with Orion).
Maybe worth filing that as a separate bug report/issue though(?) - it bugged me enough while testing that if I were to use mymind daily I would definitely want it resolved.