If it's POST then you can use the Firefox Tamper Data extension to intercept the request and modify the POST parameter values before sending it on.
The extension is a bit fiddly to get going on Firefox 29+, you need to install it, then hit Alt to access the menu bar, then go to Tools -> Tamper Data.
This will bring up a new window for Tamper Data. Fill out your form, but before you submit, click 'Start Tamper' in the Tamper Data window, then submit the form. Tamper Data should pop up a dialog asking if you want to tamper, click the 'Tamper' button and a new window containing the details of the request will appear. On the right hand side you should see your POST parameters and be able to edit their values. Click OK to submit the modified request to the server, then click 'Stop Tamper' in the Tamper Data menu.