The latest statistics on Semagic downloads say that Firefox is the most popular browser (among Semagic users). So I created a Firefox extension that adds context menu item to copy link or text to Semagic. It behaves in the same way as IE version does. Download it here (left-click): http://semagic.sourceforge.net/copytosem
It will prompt to install it at the top of the browser window. After installation and browser restart you should open Tools/Addons, find addon, select its settings and browse for Semagic executable C:\program files\Semagic\LivejournalU.exe.
click with the extension on a Leave comment link (with ?replyto= ), Semagic will switch into comment writing mode i.e. will remember the link, when you click "Post" it will post reply to that link and open web page with resulting comment. Comments are enabled only in the new version of Semagic (18.104.22.168). Also this version can send personal messages (in Links menu and friends window) and shows inbox contents.
There is a way to do the same for Opera, requires manual editing of its menu files
There are no such easy way to do the same for Chrome (because it requires to run external application).
How do you think post options should be oriented by default (for new users)?
A reminder how to post youtube videos:
copy URL to youtube video to clipboard and press button (with disk) on the toolbar in Semagic. This will add lj-template tags around video link. For other video sites, find <object code, copy it to clipboard and press the mentioned button. This will add lj-embed tags. As far as I know, lj-template tag (automatic generation of embed code) works only for youtube. Embed code for youtube also works but lj-template allows you to manually edit the links later.