Well Opera 26 seems to load my old .mht files saved with Firefox correctly. I tried right-click, open with, choose Opera from the list and the file loads properly.
Also, you can change the .mht to .mhtml and it will load into Opera 26.
The BIG problem is that Opera 26 doest't save web pages in either "Web page complete" or "Single page" mode!
If you save a page in "single page" mode, the file will not have the page graphics saved in it and if you save in "web page complete", the file will load into FF or IE but the it is just some html text and nothing else. It does save the graphics in a subdirectory but the pointers in the html file never find them.
Too bad. This makes this browser totally unusable. I can't work without being able to save & retrieve complete web pages.
Not ready for prime time!