comfortable has 8 distinct synonyms spread across 2 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is uncomfortable.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
providing or experiencing physical well-being or relief (`comfy' is informal)
“comfortable clothes”
free from stress or conducive to mental ease; having or affording peace of mind
“was settled in a comfortable job, one for which he was well prepared”
in fortunate circumstances financially; moderately rich
“they were comfortable or even wealthy by some standards”
2 of the 5 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for comfortable.
Synonyms, antonyms and the broader and narrower relations for comfortable come from Princeton University’s WordNet. Full sources and licences.