Recently, I've been getting the bad request error on Firefox when I try to visit certain pages, usually after submitting something like a comment form or an e-mail. As a result, I had to open up safari, and redo all my comments and e-mails (because it just wouldn't work in firefox!). But turns out, that there are several reasons why the bad request error might pop-up for you. I've mentioned each of the solutions as well as the solution for my particular problem.
Oh and I must also mention that I was getting the 'Opps... the system encountered a probem (#103) or (#707)'.. and so on when I tried to access Gmail through firefox. (Of course, the gmail problem can be cured more easily by just going to https://mail.google.com as Google recommends) but the bad request error will still persist.
Turns out, the reasons for the bad request problem as well as the gmail problem can be 2 main reasons:
Problem with cookies on your browserMost often it might be a problem with the cookies in your browser (or your browser not being able to write cookies to your computer). You might want to delete the cookies (Tools > Clear Private Data > Check 'Cookies' > Click ok). Or make sure that cookies are enabled, in your firefox preferences.
Problem with some extensionThere might be some extentions/add-on on firefox that you might have installed that's causing the problem. If you're using Windows try running firefox in no-addons mode and see if that fixes the problem. If you're using a Mac, you'll just have to get rid of most of the extensions (I suggest doing this one by one).
Problem with not enough user privileges to write cookiesThe problem in my case was that I was using a Mac with a user account with disabled admin privileges (I was experimenting with Parental Controls on my own account... more about that later) and as a result having parental controls enabled on my account meant that firefox couldn't write any cookies on the computer. So, it just gave me the bad result error everytime. Switching back to regular accounts fixed the problem.
So finally, I can still use firefox as my main browser on a mac.
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