While it's only minor, I find it annoying that neither of these sites had an option to add a search to the Firefox search bar, so I did some searching and found these. Open up your text editor, and copy and paste the following:

Space ALC

Save this in a file as spacealc.xml.

ニコニコ動画

Save this in a file as nicovideo.xml.

Move both of these files to your Firefox searchplugins folder. (On Linux, this should be in ~/.mozilla/firefox/[random_string].default/searchplugins/)