Although it is not the reason for the delay, you should not run a web browser as root. A web browser is a very
high-risk application from a security point of view, and you should never run one as a privileged user. For a discussion of the reasons for this, please read this post
As to the reason for being slow... It could be that there is some junk in the configuration of the browser - corrupt or messy cache files, history etc. Maybe some extensions which are not behaving well.
To check if this is the problem, go to the root home directory and rename the .mozilla directory to something else. First close/kill all instance of firefox which are running as root. In a terminal logged in as root (# is the prompt here):
# mv ~/.mozilla ~/.mozilla.old
Then start firefox and see if it works OK.
If not, and you want your old bookmarks and so on back, just kill all firefox instances and then do this to put back the original .mozilla directory. The "cd" just makes sure you are in the home directory of the root user):
# rm -rf ~/.mozilla
# mv ~/.mozilla.old ~/.mozilla
If that helps, or even if it does not, simply stop using a browser as root. You will not suffer the slightest inconvenience from using a browser as a regular user, and will significantly improve your security situation.
By the way, you should not log in to your graphical environment as root either. Re-read the link I posted at the top of this message - it is in response to someone who was asking about logging into their desktop environment as root.