A typical Apache weblog contains all the relevant user OS and browser make/model info. If it's in the logs, you'll be able to construct a search in Splunk for it without the use of a special 'Apache' Splunk app.
The pertinent user agent information in an apache log looks something like this:
"Mozilla/5.0 (Windows; U; Windows NT 6.0; en-us; rv:188.8.131.52) Gecko/20100401 WINFC 184.108.40.206 Firefox/3.6.3 (.NET CLR 3.5.30729)"
Mozilla/5.0 is the family of browser (the 5.0 is some legacy netscape business)
(Windows; U; Windows NT 6.0; en-us; rv:220.127.116.11) is the general OS and build information: OS, Country Code, Platform, Language Variant, rendering engine version number
Gecko/20100401 is the rendering engine and build date
Firefox/3.6.3 is the actual browser version used
If you're splunking all of your Apache logs, I believe 'useragent' should be an automatically extracted field. Check the 'other interesting fields' section in the light blue field-picking pane within the search app. Clicking through the useragent field will give you the most granular expression of this information, but you'll want to present your results in a much more general format to make an adequate comparison of browser types.
I would start the search with eventtype=pageview to restrict the results to only pageview activity. From there i'd be comparing the most general terms - events that include 'firefox' 'safari' etc.
This probably doesn't answer your question since I can't think of the correct search off the top of my head, but hopefully this gives you some context and points you in the right direction.
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