Stone carvings from a synagogue that existed at the time of the Temple depict the Menora with curved brances.
See http://www.nbcnews.com/id/32797473/ns/technology_and_science-science/t/-year-old-menorah-depiction-found/ for details.
Despite the claims of Chabad, also Rambam and Rashi agree with the shape of the branches, depicted on the stone carving from the 2nd Temple period that was photographed in the article I linked to.
On the assumption that the links don’t change, see http://www.hebrewbooks.org/pdfpager.aspx?req=37948&st=&pgnum=373&hilite=
and see http://gallica.bnf.fr/ark:/12148/btv1b84700460/f233.image.r=H%C3%A9breu.langEN
as evidence that both Rambam and Rashi accepted a curved branch Menora.
The synagogue that Rambam prayed in, also contained depictions of a curved branched Menora.
See http://www.nbcnews.com/id/32797473/ns/technology_and_science-science/t/-year-old-menorah-depiction-found/ for details.
Despite the claims of Chabad, also Rambam and Rashi agree with the shape of the branches, depicted on the stone carving from the 2nd Temple period that was photographed in the article I linked to.
On the assumption that the links don’t change, see http://www.hebrewbooks.org/pdfpager.aspx?req=37948&st=&pgnum=373&hilite=
and see http://gallica.bnf.fr/ark:/12148/btv1b84700460/f233.image.r=H%C3%A9breu.langEN
as evidence that both Rambam and Rashi accepted a curved branch Menora.
The synagogue that Rambam prayed in, also contained depictions of a curved branched Menora.
New Addition: I have seen one ancient manuscript that makes me a bit less sure about Rambam’s opinion on this issue. But I remain completely convinced about Rashi’s view on the subject.
In the standard Shas Vilna edition of Menachot 98b the following shape is attributed to the Menora by the main commentary on the page.
