How to install extensions on Android's stock browser? I have JellyTime (R30 - https://code.google.com/p/jellytime/) installed on my HTC Desire HD phone which is running a variance of Jelly Bean 4.1.2.

Is it possible to add some extensions/features to the default available Internet Browser in Jelly Bean? For ex. Ad Blocker, Flash Blocker, etc.?
The default Browser does not have extensions, but what you are looking for might be found as Android apps, for instance: "Bookmark Sort and Backup" (BookmarkSB).   The bookmarks themselves are considered part of Android not of the default browser, but the addon would only have use through the default browser and there are changes made to the options in Browser that you will not see in other browsers from installing this app.

For serious stuff like effective ad blocking and changes to the hosts file you would need root access, before you can use full features of such addons, and in particular modify your hosts file.   The hosts file would apply to your entire system not to a specific browser or application.