Loading a world prevents any open browser pages from refreshing, and no new pages will load.

Instant message clients continue to function, and closing them does not affect internet performance.

This happens in both singleplayer and multiplayer.

Returning to the title screen does not allow pages to load once more. Minecraft must be closed entirely.

I have a lower-end PC, but this really shouldn't be an issue. No other program/game I use interferes with web pages in this manner.