This applies to M11 for linux. This could be a wierd interaction with X. One symptom (I beleive) is that the first "Ctrl+N" does not open a new browser window. Another is that entering URLs fails sometimes. Another is that passwords are often incorrect (ie, typing "password" results in "assword" being input. This is reproduceable after any mouse button is pressed.
I think this is a dupe of bug #19058.
*** This bug has been marked as a duplicate of 19058 ***
[bugday] dupe. I read both. Verified others just like this too. :)