I made further experiments and partially solved the problem:
Start the (UMTS) modem, start Firefox, select "File" -> "Work Offline". "Work Offline" should now be unchecked and Firefox should now work properly.
Interestingly, most of the rest of my system reacted on this change, too. Adept and Evolution both work. Konqueror, on the other hand, still refuses to recognize the connection through the modem.
I made further experiments and partially solved the problem:
Start the (UMTS) modem, start Firefox, select "File" -> "Work Offline". "Work Offline" should now be unchecked and Firefox should now work properly.
Interestingly, most of the rest of my system reacted on this change, too. Adept and Evolution both work. Konqueror, on the other hand, still refuses to recognize the connection through the modem.