You could setup your framework project as a submodule in your prja and prjb projects.
From the git book:
Git's submodule support allows a repository to contain, as a subdirectory, a checkout of an external project.
From the root directories of prja and prjb just do:
git submodule add git://path/to/your/fw.git path/to/fw
The first path is wherever your fw git repository is, and the second is where you want fw to go in the project that is using it.
More info from the Pro Git book:
It often happens that while working on one project, you need to use another project from within it. Perhaps it’s a library that a third party developed or that you’re developing separately and using in multiple parent projects. A common issue arises in these scenarios: you want to be able to treat the two projects as separate yet still be able to use one from within the other. [...] Git addresses this issue using submodules. Submodules allow you to keep a Git repository as a subdirectory of another Git repository. This lets you clone another repository into your project and keep your commits separate.