I've been using Firefox on Linux for quite some time. My default search engine is DuckDuckGo. I haven't had any problems with Firefox.

My wife uses Firefox on Windows 7 (she hated Windows 8, so we switched back to Windows 7) and she had been having some problems with Firefox (can't connect after auto-upgrade, Firefox using lots of CPU resources, etc). I finally figured out the connection problems (it was our Norton firewall needing to be told about the Firefox update), and doing a Firefox refresh (https://support.mozilla.org/en-US/kb/refresh-firefox-reset-a...) seems to have solved the excess CPU usage problems.