Remember when an apps politely asks you if you want to install their (or their partners) toolbars and change the default homepages on your browser? Pepperidge Farm remembers.
Errmm okay, this was happened days ago when I installed some random freewares and somehow it's messing up my browser configurations, I don't remember ticking any 'extra features' on the installations but ended up changed things anyway on my Firefox. At least I have http://search.snapdo.com/?st=ds and http://searchiu.com/?affil=141 listed as the default homepages and search engines.
Now removing the snapdo it's a lot easier, I just have to changed the homepage back to default and remove it from the search engine list. I remember got the another one after installing a GrooveShark downloader (which I uninstall minutes later because it doesn't work, hooray!). It's more harder to get rid of, as it will restore its settings again after you removed it.
It seems that it created a custom script to undo my changes, so I have to remove it first. Here's what I've done :
1. Open 'Troubleshoot Info' on Firefox (Firefox -> Help or type "about:support")
2. Scroll it down until I find entries that stated something created a user.js script
3. Open the user.js link and confirm the contents related to the search engine
4. Navigate the location on the address bar with Explorer and delete it
5. Undo the damages : restore my homepage, remove it from search engine list and restore what Firefox will do if I open a new tab.
*As for the last thing, it seems there's no easier way other than change it via preferences page :
1. Type "about:config" on Firefox
2. Click "I'll be careful, I promise -whatever"
3. Type "browser.newtab.url" on search box
4. Click on the only search result
5. Change its value to your preferences (the default is "about:newtab")
Works on Firefox 23 (Windows 7 x64).