Comment # 16 on bug 1227486 from Fabian Vogt

I'm mostly out of ideas. The only way this can happen AFAICT is if KIO thinks
there is no browser available so it tries to download the file from that URL to
figure out which application to use. The xdg-mime and kreadconfig6 calls show
that the browser is correctly configured though.

At this point I'd treat this more like a normal bug report than a security
issue, but I still can't tell for sure whether this is a bug just on your
system or whether it may happen generally.

Some more invasive debugging might be needed. You could attach to the running
kmozillahelper used by TB and set a breakpoint on "KIO::get". If that triggers,
the backtrace might be helpful