Firefox Add on messing up gMail

There is an extension in Firefox that messes with my gMail, but I am too lazy to search one by one to discover which one is it.

Actually, I think I already identified it, it must be either Web Developer or Tab Mix Plus (which by the way is not yet available, at least from the official site, for Firefox 3 and this is why I haven't installed the latest version of Firefox on my main PC).

The problem is when it comes to download things; while downloading files, sometimes (I think always or at least very often) when I try to download an attachment from a message the page goes blank... you can download the file but then the window doesn't display anything (like if it was an empty website, in white background and no text). Firefox continues to work normally, I can switch tabs and everything, even if I click on reload, it goes to that very same conversation (message), but this is not supposed to be and is very annoying.

I thought it was a problem with the not so new gMail interface, but the other day I was browsing in a computer from the school and I noticed that I didn't have that problem there, so I assumed it must be an extension.

Then, in another PC, which had installed only these two aforementioned add ons (which are essential for me in Firefox), the problem was there again. I think I should send this message to the developers of these plugins, maybe later on.