EDIT: Have you tried the options menu yet? Not sure if you saw this reply.

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There's a short text that tells you useful things after installing the addon, but you might not have seen it, so: SSS has a TON of options that you can change! ;)

Opening searches in a new tab instead of the current one is changed in the "Popup/icons behaviour" section. Please look for "Left mouse button click" to change this behaviour. SSS opens in the current page by default since it's what Firefox does by default as well.

Regarding the back button being unavailable when opening a new search, SSS shouldn't cause this and I had no other reports of this problem before, so I suspect that some other addon or a Firefox setting could be the culprit in your case. I suggest trying to disable your other addons and/or run SSS in a clean Firefox install to see if the problem persists.

I hope this helps!
Daniel