To manage application preferences

Hei;
the question is NOT how to change default application etc

I know that "right click, get info, open with..."

But what I must do when the application I want to use to open file is grayed out?

More specifically - usually I read mu .epub-s in calibre (default application), but sometimes I'm using Epubreader (Firefox add-on). It works fine to open ebooks via Firefox "open file...", but could be fine to right-click file and point Firefox sometimes... Anyway, I can't choose Firefox with "open; other..." command.
Any way to 'teach' to mac that it's ok to open .epub's with FF?

Sincerelly -
AM

oops...

stupid me...
Thanks!

Thanks, but...

Thanks!
Yes, that is good point ;-)

But it only means, that I was not exact. Yes, that works fine - change 'recommended applications' to 'all applications' and ok, but what I really like to do is to edit that recommendations list - whatever or whereever it is.
Like, f ex, when I right-click on .jpg, there are default application (Adobe Photoshop), but there are also long list of applications, which potentially can open that file (Illustrator, Firefox, Safari, Opera, Neooffice, Previw etc etc). How can I tell to OSX, that it is ok to open .epub with Firefox -- so that FF will appear in that list?

Tnx again-
AM

OK, it seems to be quite meaningless forum, since I have had no meaningfull answer. But in case if somebody finds this post f ex google search --
This problem is basically issue of MacOS Launch Service.
Usually it helps, when rebuild LS database, but as long as developer have not added Launchservice information, you must do that by hand. Best way is to isntall RCDefaultApp.
Thanks for no help!