I use iMacros in Firefox to prefill forms when testing. It works very well and is easy to use, but it uses field IDs which change every time you make any change to a field in Composer which breaks the macro.
The experimental event recording method doesn't have this issue, but it makes it much harder to edit the macro scripts, which I tend to do quite a bit to just cut out lines or change data.
What do other people use to fill forms when testing?
Another one of our clients uses the attributes data-sfc-name and data-uid in their automated test scripts. These attributes only change if you change the name or path of the field in composer.