@videobruce:
1.
Tabs Mix Plus: Firefox's and SeaMonkey's tabbed browsers are still too different for this extension to be easily ported, it would amount to a full rewrite. The author is not interested, and Philip has already said it would be too much labour. All this has been mentioned several times, including in this very forum topic. However, there is work being done on the SeaMonkey trunk code to make its tabbrowser similar to Firefox's so that extensions may be ported more easily. Far from done, though. You may want to follow
bug 558614 and
the other bugs it blocks. Once everything is FIXED, try porting the extension yourself: loud mouth no hands don't get the job done. (Just kidding.)
2.
Additional buttons: On SeaMonkey 2, toolbars are now customizable, just as in Firefox or Thunderbird, and the way to do it is the same: right-click any toolbar, then Customize; then move buttons about by drag and drop. You may want to make sure first that none of your toolbars are hidden. If the button you want is not available in the plain-vanilla Suite, look for an appropriate extension by searching
https://addons.mozilla.org/en-US/seamonkey/. There are many extensions which provide additional buttons, the only problem is to find one that fits you — or to write one yourself.
If you mean the "Site Navigation" toolbar, it is dependent on which
<link> tags have been written as part of the
<head> in the current page's HTML: I see Top, Up, First, Previous, Next, Last, Document, More, and RSS/Atom:Subscribe as possibilities, but it's up to whoever writes the page's HTML to provide the links. Works quite well for the pages I write myself and load via a
file:/// URL.
3.
Slide show: Not sure what you mean or how to provide it.