Ghostery is open-source for almost an year now, You can check the code and repo: https://github.com/ghostery/ghostery-extension

Unless you audit the source of every piece of software you use as well as that of every compiler used in making their binaries, you aren't in a position to make that sort of absolute statement. OSS has benefits but for most users they are delegating the code review to someone else, which makes it very similar to proprietary code.

There is an argument to be made about motives with free vs non-free software, but open vs closed source is for most people much less important.