The term 'root firmly' can be summarized by two crossword solutions: ENTRENCH (8 letters) and EMBED (5 letters). ENTRENCH means to establish something firmly or deeply, while EMBED means to fix something firmly in a surrounding mass. Both terms refer to the idea of setting something in place and making it difficult to move or remove. This is often used in the context of planting a tree or shrub, where the roots must be firmly established in order for the plant to survive.