When a save/open dialog is called for a download and its originating tab is closed, it's not possible to download the file anymore. This used to work in 16.x, now it doesn't work anymore in 17.x, Aurora and Nightly.

I just did better tests with a clean profile every time and all versions up to the latest Nightly work as expected. Before I was testing with Portable Firefox from PortableApps.com and I can still get the same results with any version I throw at it, maybe a profile generated from it can help determine what is causing the problem. I just want to make clear that my non-portable Aurora profile that I have been using since 4.0 has the same problem. So, disregard the last paragraph of the previous comment, the functionality works as expected with a clean profile.

It seems that setting browser.download.useDownloadDir to false in a clean profile and restarting Firefox causes the problem, that's one of the default settings from Portable Firefox, so now it's understandable why the problem always happens with it. I can confirm that saving and opening still work after the tab is closed when the setting set to true, but other things like FlashGot don't.