One of the most useful tasks that we rely on the community for is testing and reporting of bugs. Since Xfce runs on various platform and in a lot of different setups, testing all changes in every possible situation is an impossible task. As such we kindly ask users to assist in testing, and reporting all bugs they may find, using our bug tracker.
Once a bug has been found, the cause of the bug needs to be tracked down, and then (obviously) fixed. If you want to get involved in the actual development process of Xfce a great way to start is by solving bugs and then submitting a patch file.
Although the philosophy of Xfce is to find the correct balance between features and lightweight, it is still possible there are features you'd like to see in future releases.
This right approach for larger changes is to discuss them on the mailing list first. You might think your idea is brilliant, but there is a high possibility other users.
Afterwards a bug can be opened in the bug tracker and make sure the Importance is set to normal and enhancement.
Obviously its also nice if you can write a patch that implements the feature, but its not a necessity.
We always try to make the software as stable as possible, but there is always a possibility a crash or segfault can occur. To problem with crashes is that they are not always reproducible by the developers, this makes it quite often hard to track-down the problem and patch the issue.
Nonetheless there are a couple of things you can do to help resolving them:
Search the bug tracker and look if the issue is already known. Also look through the resolved bugs because the issue might be fixed already, but has not been released or applied downstream. Peeking the GIT repository might help for the latter.
If you have not found anything or missed it, report it in the bug tracker. Reports that only say 'application X crashed' will be closed as invalid quickly, so try to provide a way to describe and reproduce the problem: Application X crashed when doing Y.
In the description provide a short way to reproduce the crash. Also run the application in a terminal and look if there are console warnings, put those in the bug as well.
If the developer cannot reproduce it, he might ask for a backtrace, read more about creating backtraces here.