Add an option to turn off slide animation

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Have Hide BookmarksBar addon installed
2. Turn on "Hide bookmarks toolbar when a page is opened" (tick checkbox in 
addon preferences)
3. Open a page from a new tab or homepage

What do you see?
Bookmarks toolbar slides "under" address bar.

What is the wanted output? 
Bookmarks toolbar disappears instantly or after page is loaded.

What version of the product are you using? On what operating system?
Hide BookmarksBar 3.3
Firefox 35.0.1

Original issue reported on code.google.com by [email protected] on 9 Mar 2015 at 12:22