Firefox wants to download admin.php when using MT upload script

I saw this (or similar problems) mentioned several times when I search, but couldn’t find a resolution. I’ve installed WP on my home Ubuntu server (Dapper). Here’s the sequence of events.

1) Tried to import some old Movable Type posts. Got the following error message:

Unable to create directory /var/www/blog/wp-content/uploads/2007/01

2) Changed the permissions on /var/www/blog/wp-content to 777

Tried again, and got (one time) an error message about exceeding memory allocation (or something, can’t reproduce it now). I assume this means that with a server with only 512 Mb of memory, I have to chop my MT import file into smaller pieces.

Tried again with a smaller file. Firefox asked me what program I want to use to open admin.php.

Changed the permissions back, just in case.

Tried again. Firefox still asks me what program I want to use to open admin.php.

Help!