Add getFindBar for Fx extension

[BMO: 1006714: *'Find' fails to search whole PDF with pdf.js when opened in a new tab*](https://bugzilla.mozilla.org/show_bug.cgi?id=1006714) was caused by [BMO: 537013: *The find bar should exist on a per-tab basis*](https://bugzilla.mozilla.org/show_bug.cgi?id=537013) which made `gFindBar` point to the *active* tab's findbar.

This pull request addresses the defect by adding a function to get the correct findbar based on the `domWindow`. It also adds a test that checks if the findbar has been initialized (I verified that the test fails pre-patch and passes post-patch, in addition to testing that the behavior described in the bug is fixed).