There are several synonyms for the phrase "fix firmly," each with their own subtle differences in meaning. The most common synonym is "secure," which implies making something stable and safe. "Anchor" implies making something immovable or stable, as if it is anchored to the ground. "Fasten" suggests attaching something to an object or surface, making it secure and steady. "Affix" implies attaching something to something else, usually using adhesive. "Plant" suggests firmly establishing something in place, as if it is planted in the ground. "Embed" implies something is deeply fixed into a material, creating an inseparable connection.