Rated 3 out of 5 stars
I cannot install this addon either,because of an error message: A better firefox addon bar cannot be installed because Firefox cannot modify the needed file.
I try to search in the Internet,I find that may be the answer:
It seems that some intermediate file of the installation process is being created with a name longer than the maximum supported length and then the installation process fails.
Your add-on name may be too long in the internal.
Cannot modify the needed file fix
Please go here to find the solution to your problem, http://support.mozilla.com/en-US/questions/818704
If you are still having problems, leave me a comment here:
Also let me know if there are any other details related to the error you are experiencing, thanks!