Sometimes applications stay in the background and have an empty white arrow in the launcher. How do I fix this?

Some applications hang, as a result they preclude this behaviour. I found this exists in thunderbird and firefox.

I recommend using a little app called FlashFreeze.

Install

FlashFreeze works in Ubuntu 11.04 and 11.10 and can be installed by adding the following PPA (Personal Package Archive) to your Software Sources: -

ppa:shnatsel/flashfreeze

After adding the PPA run an update sudo apt-get update and install ‘flash freeze‘ from the Ubuntu Software Centre.

Here is the official documentation for the open triangle:

Single open triangle: the application is running in a different workspace (by default, Ubuntu 12.04 sets up 4 workspaces).

Except that I have been also experiencing this and the application is under no workspace. Which is probably a bug, this one, or one of its numerous duplicates.