I noticed  a problem with gallery slideshow, and then tested with some web browsers:

Firefox  (45.0.2)  = when starting slideshow, a new tab opens and timer counts to 0 but then nothing, the whole window is freezed
IE (11)                   = works ok
Chrome (49.0.x)  = works ok
Opera (36.0)        = works ok

It may be possible to get slideshow to work with Firefox too. However I tried it in two different computers and tried some settings, no luck.
Maybe someone can give light, how is Firefox different to the other browsers?

Thanks for the report. I can reproduce, there's an "undefined entity" error in the browser console also. Investigating…

Ok simple fix…

In site/pages/modules/galleries.php change text/xml to text/plain.

Thanks again Chris,

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