it does also work on Linux (Ubuntu in my case) in the firefox pdf reader
1) Do Cmd+A / Ctrl+A
2) Open up a smaller program that will be on top of the firefox page but won't cover ( e.g. a terminal ) so that firefox is out of focus
3) The blue bar should have the answer under.
If it still does not work maybe ask a friend who either has Linux or Windows.. or use a VM
here is an example in an image.
