What web mail provider are you using?

GoDaddy. I spoke with them this morning and they said they had customers call with the same error. I'm on a Mac and I tried doing the same thing on a PC with a different version of Firefox and it worked like a charm. It works ok on Safari, bit I don;t want to have to switch to Safari, as I'm sure Firefox doesn't want me to either.

Troubleshooting extensions and themes

• https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes

Clear Cookies & Cache

• https://support.mozilla.com/en-US/kb/Template:clearCookiesCache

Clear the Network Cache

• https://support.mozilla.com/en-US/kb/How%20to%20clear%20the%20cache#w_clear-the-cache

Troubleshooting plugins

• https://support.mozilla.com/en-US/kb/Troubleshooting%20plugins

Check and tell if its working.

I would follow the advice provide to you by mha007. I have a Godaddy email account and I was able to login and upload an attachment to include in an email message using Firefox 6. You could downgrade but it's not recommend.