When attempting to add a custom search engine in Firefox Android with the inclusion of a ":" character in the query portion of the URL, the following error message occurs:
Error connecting to "[UserInputtedSearchEngineName]"
The problem can only be remedied by replacing any inclusion of the ":" character with "%3A", which is the HTML URL Encoded version of the character.
The same behavior does not occur when adding a custom search engine in the desktop version of Firefox or Chrome, patching the behavior would aid in the usability of the Firefox Android browser.