See the URL. After you follow the link to the text document and go back, View Source is disabled. The same holds for coming back from images and PDFs. r14376 is when this regressed for text documents. Images were still using WebImageView at the time.
*** Bug 12360 has been marked as a duplicate of this bug. ***
From bug 12360: <rdar://problem/4944800>
Created attachment 13238 [details] Update the FrameLoader's MIME type for pages coming from the b/f cache
Comment on attachment 13238 [details] Update the FrameLoader's MIME type for pages coming from the b/f cache r=me
Landed in r19715.