Thanks, this gave me an idea where to look. I finally managed to fix my Firefox browser. There are a few steps I didn't see mentioned. On the config screen (about:config) there are two more entries that need to be reset apart from browser.search.defaultenginename. When making a search I found the following entries:
keyword.URL;http://dts.search-results.com/sr?src=ffb&appid=0&systemid=102&sr=0&q=
extensions.wrc.SearchRules.rambler.ru.style;.WRCN {display:none} .search-results .title + .WRCN {display:inline !important; background: url("IMAGE") right no-repeat}
resetting keyword.URL and extensions.wrc.SearchRules.rambler.ru.style fixed my problem.
Strange, though, going back into about:config after having reset those two, I can't find the second entry (extensions.wrc.SearchRules.rambler.ru.style) at all anymore, only
extensions.wrc.SearchRules.rambler.ru.URL