@Runjini Place the Browser filter on
Viewed Web Page activity, not
Clicked Link in Email (you can add
Clicked Link as an additional constraint).
The Browser box will still be free text, but it will indeed search in the User-Agent string for your page views. For example, if you enter "MSIE 10.0" it will find hits from IE 10 (or browsers pretending to be IE 10, but that's another story).
Note that User-Agent parsing can work fine for determining the lion's share of browser identities (people who fake their U-A may not be people you really care about) but the problem is that as far as I know Marketo applies no deeper intelligence to the search. For example, if someone has Chrome they will match on a search for "Safari" because Chrome also includes the word "Safari" in its U-A to encourage compatibility. A real User-Agent parser, on the other hand, uses a huge list of in-the-wild identities and can distinguish browsers who happen to include a certain browser's name from browsers who really
are that browser. If you wanted to add this capability to your Marketo database, you could use a
User Agent String API to look up the real browser identity once per session and store it on the lead.