I can't reproduce 100% of the time. When starting at a page with a horizontal scrollbar and then switching to a page that doesn't need one, the scrollbar doesn't disappear. Start at https://bugzilla.mozilla.org/show_bug.cgi?id=302212, bug 302212 comment 2. There's a long link in that comment. It's so long that the window will get a horizontal scrollbar. Click that link. The target page loads, but the horizontal scrollbar doesn't disappear. It should, because the target page isn't wide enough to need horizontal scrolling. Use the down arrow to scroll down in the new page. Note how the content area has expanded to encompass the scrollbar that should have disappeared, so the scrollbar is scrolled as if it were part of the page. This may be dependent on window geometry. On a 1280x854 display, I'd estimate my window size at 1150x775. Tab bar and find bar are open.
Related to bug 278812 ?
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