User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

LINUX:

1. Open FF Settings, browse to Applications
2. Search for a file type e.g. PDF
3. Set Firefox as application to handle the file type (NOT: Preview in Firefox)
4. Open a file of chosen file type from file browser or Internet

OR

1. Open a file from internet, so that the "Open With" dialog appears
2. Choose Firefox via Open With and check "Do this automatically...", confirm

WINDOWS:

1. Choose FF as default app for e.g. .pdf-Files
2. In Settings->Applications search for e.g. PDF
3. Set TWINUI (default app) to handle file type
4. Open a file of chosen file type from file browser or Internet

OR via "Open With.."-Dialoge, see above

Actual results:

Firefox opens a lot of tabs containing the specified file and has to be forcefully quit to abort the opening.

Expected results:

Firefox opens one tab or one window containing the file