WP site with Firefox Admin issues on OS 10.6.8

I work on a number of WordPress sites, but this one has come up with a crazy problem on Mac laptop running OS 10.6.8.

This began when I was using the wi-fi at a sports center, meeting with a client and trying to add a plugin. No password was called for, and an unfamiliar server. That is when I started getting “406 Not Acceptable” An appropriate representation of the requested resource /blog/wp-login.php could not be found on this server.”

I followed up on some support threads, and found one which suggested the hosting might have some sort of issue, in this case Bluehost. Their support said it was a plugin issue. I then looked at the site with Filezilla and discovered the plugin I had been trying to add was in the plugin folder, having been installed in spite of the message.

Then I discovered that my limitations had become progressive, now making it impossible to switch to other Dashboard areas like Appearances, now also blocked with this message.

I tried switching to another computer and discovered that all of the admin, including plugins are fully accessible and functional, so this is just affecting this Mac in Firefox.

I cleared the cache and cookies. This made it worse. Now when I login the admin has no CSS and I am presented with a white screen with an unstyled list of the text links for the admin dashboard. If I click on one I continue to get the error message.

Help! I have worked on a lot of WP sites and have never seen this one. How can I get rid of the error message and regain the styles for Admin?

Thank you