Louis Gray asks an interesting question in a post on his blog: "Is requiring a Plug-in akin to designing for IE Only?" Like Louis, I use the Safari browser for a lot of my browsing, which doesn't support plug-ins. The rise in popularity of Plug-ins for Firefox has occurred at the same time as the rapid adoption of the iPhone.
The iPhone is in the vanguard of mobile internet use. This trend will continue with the introduction of Android phones and the continued popularity of the Blackberry. The Mozilla foundation is working on a mobile version of Firefox but this is not a real solution. Mobile phones are not the only way we access the Internet. Games consoles and Internet Tablets, like the Nokia N810 are also entering the mix.
I believe that the future lies with developers creating cloud-based functionality that can be activated from a bookmark. Examples of this already exist with bookmarklets from services like delicious and bit.ly. The advantage of the bookmarklet is that it can be designed to be browser-platform independent and can also deliver functionality to mobile devices.