The project runs Git-flow, where the master branch is the production one and the develop is the developing one.
In a nutshell, if you're about to propose a new feature with adding new functionality to bowser, it's better to branch from develop and make a PR pointing to develop as well.
If it's a small hotfix, fix a typo in the docs or you've added support for a new browser/OS/platform/etc, then it's better to branch from master and make a PR pointing to master as well.
Following these simple rules will help to maintain the repo a lot! Thanks ❤️