Firefox doesn;t either IIRC. The only 'modern' web browser I know of that does is Internet Explorer (I use the term modern loosely when referring to IE). Still, you have to click that setting when the download dialog pops up.
Firefox does allow this, at least the way mine is set up, not sure if I had to do something years ago in About:config to get it to work, but I know that it's not via an extension.
Besides, is it really that difficult to double click the download dialog in Opera when its finished? It really takes you 6 hours a month to double click a box? Its literally 1 extra click vs IE.
Maybe I'm misunderstanding the workflow, and I'd appreciate it if there was a better way to do this, but at the moment it seems like a lot more than one extra click.
Workflow in Firefox:
Click on file you want to open
Click open in dialog box vs. Save As
Occasionally clean out temp folder
Workflow in Opera V1:
Click on file you want to open
Navigate to your designated temp folder where you want it saved (possibly a lot of clicks) and save the file
Wait for it to download
Open up the download manager in Opera and double click the file you wanted to open
Occasionally clean up your designated temp folder
Workflow in Opera V2:
Have Opera automatically save everything to one common download folder
This saves some time when you're trying to open a file, but it seems that it is more than made up for when you have to try and clean up your downloads folder and see what is worth keeping and/or moving to a more permanent location, and what is not.
I go through phases where I open a lot of VCF files. Maybe just to get a quick piece of info like an email address, or maybe open it and then save it to Outlook. Either way, the VCF file is useless to me about two seconds after I open it. I just don't want to have to think about it again. It would be slightly more bearable if when you cleared the download list it actually gave you an option to delete the files at the same time, but without that I end up with a download folder with thousands of items in it, about 10% of which I actually want to save, and the rest that I don't.