I've tested this with Chrome and Firefox. In Chrome all is fine, in Firefox I've got the problem:
You can test it right here: http://getbootstrap.com/javascript/#modals-related-target . If you open one of the modals, focus an input, enter some characters, switch to another Firefox tab or to another window and then come back to the tab with the open modal, the input is not focused anymore. The user has to click again into the input. In Chrome the input is still focused, in Firefox not.

Can you recommend a workaround for this?