Applying a new theme from the preferences menu resets the browser, losing whichever page that you were on. Seems like the expected behavior would be to reload the page, keeping history, etc.
Using Build 2000082921
Dup of bug 44437 and bug 47345. Marking dup of former... *** This bug has been marked as a duplicate of 44437 ***
Verified as a dupe of bug 44437 "Current web page disappears when switching themes"