You could do something like this. I'm using a static list of all possible browser in the eval-case command. If your base browser name follows some pattern, you could extract that and assign category.
index=sccm sourcetype=SSCM_BROWSER_DEFAULT | stats count by BrowserProgId00
| eval Browser=case(match(BrowserProgId00,"Chrome","Chrome",
match(BrowserProgId00,"Firefox","Firefox",
match(BrowserProgId00,"Safari","Safari",
....add more browser values per your data...,
true(),"Unknown")
| stats sum(count) as count by Browser
... View more