Was checking out this blog's visitor stats recently when I came across this User Agent "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.2) Gecko/20090729 Firefox/3.5.2 GTB5". I was wondering what GTB5 was. Some googling and testing later I found out that GTB is in fact the Google Toolbar. To compare, here's the UA of a Firefox browser with Google Toolbar installed :
"Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.2) Gecko/20090729 Firefox/3.5.2 GTB5"
And here's another Firefox without it :
"Mozilla/5.0 (Windows; U; Windows NT 6.0; en-GB; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3 (.NET CLR 3.5.30729)"
Also, if you plan to capture the UA string in the Apache logs, make sure that feature is turned on. Your LogFormat directive must have something like \"%{User-Agent}i\" in it. For eg,
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
You can check the Apache documentation on custom log formats for more info. If you're using XAMPP/LAMPP, you might want to comment out
CustomLog logs/access_log common
line in your httpd.conf and uncomment
CustomLog logs/access_log combined
line.