You can have all of that with just two extensions, firefox lets you know when updates are available, and installing extensions is completely painless. Just so you know...
Tabbrowser Extensions - handles just about everything on your list except for sessions (it actually does handle that, but it does it poorly, so I disable that part).
Session Saver - Has distinct files for each session, autosaves even on crashes, can be set to either autoload the last session, or let you choose a session. Also lets you load / save a session to / from a file, or stores your last 5 saved sessions in a "recent documents" style menu.
Oh, except for the memory thing... firefox usually uses about 440 megs for me, but I usually have 30-50 tabs open. Does Opera use significantly less?
Also, there are some amazing other extensions that you may want to consider should you switch to firefox.
IE View - For the occasional site that doesn't look or work right under firefox, puts an option in the right click to open the page in IE (simple, but damn useful).
PDF Download - When you click to a PDF link, it offers you the option of saving it to disk rather than opening it inside of Firefox. I really hate viewing PDF's while browsing, since it necessitates the 30 second delay of opening PDF reader (by the way, why the hell does the PDF reader take so fucking long to open anyway?).
Greasemonkey - The grand-poobah of script enhanced browsing, once you try it, it's just too cool to live without. It lets you use Jscript to modify any website you view. For instance, when I view Dailykos with greasemonkey, I see the rightmost sidebar (the one with the menu options at the top) and the text of the articles stretched from all the way on the left all the way to the sidebar. It makes it oh so much more legible... Also, for netflix, it makes your edit queue screen drag and droppable. And it adds IMDB links to each and every movie. It's awesome.
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Tabbrowser Extensions - handles just about everything on your list except for sessions (it actually does handle that, but it does it poorly, so I disable that part).
Session Saver - Has distinct files for each session, autosaves even on crashes, can be set to either autoload the last session, or let you choose a session. Also lets you load / save a session to / from a file, or stores your last 5 saved sessions in a "recent documents" style menu.
Oh, except for the memory thing... firefox usually uses about 440 megs for me, but I usually have 30-50 tabs open. Does Opera use significantly less?
Also, there are some amazing other extensions that you may want to consider should you switch to firefox.
IE View - For the occasional site that doesn't look or work right under firefox, puts an option in the right click to open the page in IE (simple, but damn useful).
PDF Download - When you click to a PDF link, it offers you the option of saving it to disk rather than opening it inside of Firefox. I really hate viewing PDF's while browsing, since it necessitates the 30 second delay of opening PDF reader (by the way, why the hell does the PDF reader take so fucking long to open anyway?).
Greasemonkey - The grand-poobah of script enhanced browsing, once you try it, it's just too cool to live without. It lets you use Jscript to modify any website you view. For instance, when I view Dailykos with greasemonkey, I see the rightmost sidebar (the one with the menu options at the top) and the text of the articles stretched from all the way on the left all the way to the sidebar. It makes it oh so much more legible... Also, for netflix, it makes your edit queue screen drag and droppable. And it adds IMDB links to each and every movie. It's awesome.