firefox: add page (#3261)

pages/common/firefox.md

@@ -0,0 +1,24 @@
+# firefox
+
+> A free and open source web browser.
+> More information at https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options.
+
+- Open firefox at www.duckduckgo.com:
+
+`firefox {{https://www.duckduckgo.com}}`
+
+- Open it at a new window:
+
+`firefox -new-window {{https://www.duckduckgo.com}}`
+
+- Open an incognito window:
+
+`firefox -private-window`
+
+- Search 'wikipedia' on your default search engine:
+
+`firefox -search {{wikipedia}}`
+
+- Launch firefox without extensions:
+
+`firefox -safe-mode`