Comments not functioning properly, different results depending on browser

Greetings,

I have been working on this problem for two days and have read countless posts and tried every suggestion I can find. When I submit a comment, whether logged in or not, I am redirected to wp-comments-post, which comes up blank. If I use Firefox from my Mac, the comments is added to the database, but if I use any other browser on my Mac, nothing is added to the database. The comments do not get added to the database from any browser on my PC.

I was using a custom theme, so I tried switching to the default theme. With the default theme activated, the comment feature works and redirects properly, from both IE and Firefox on my PC, as well as from Safari and my Mac, but fails from Firefox on the Mac (Firefox from the Mac was the only thing that worked on the custom theme).

Things I’ve already tried:
Turned off all plugins
Dropped and recreated wp_comments table
Increased comment_agent field to 512 characters
“comment_id_fields();” code is correctly placed
“echo $id;” code is properly placed and I also tried using “echo $post->ID;”

Basically, I’ve tried every suggestion in this thread and every suggestion to which that thread links.

I’m using WordPress 2.9.2 and am running MySql5. I am very new to WordPress, but I am an experienced developer with a moderate level of expertise with PHP.

I would greatly appreciate some help with this. If I can’t get these comments to work reliably, I’m going to have to drop WordPress and start over with another blog solution. I really, really don’t want to have to do that at this stage of the game.

Thanks!