Comfort
Comfort is one of the basic needs influencing a pawn's mood, as well as a stat value of furniture items.
Comfort need
The comfort need is satisfied by sitting in or resting on comfortable furniture.
The comfort need mechanic is similar to the "rest" and "beauty" mechanics: the comfort need bar of a pawn grows while they are using comfortable furniture, and slowly decays otherwise.
Depending on the comfort stat, a certain maximum level of the comfort need can be obtained. The maximum possible comfort level is the comfort stat of the furniture item in percent, capped at 100% (eg. a bed with 0.75 comfort can raise the comfort need gauge to at most 75%). When the comfortable situation ends, the need level will gradually revert back to 0%.
Comfort levels of 60% or higher give increasingly beneficial mood thoughts, starting at "Comfortable +6". A particularly low comfort level of less than 10% incurs the "Uncomfortable -3" thought and mood penalty. This usually only occurs when the pawn had to sleep on the ground without even a sleeping spot. It is, however, unrelated to the "Slept on ground -4" negative mood effect (which would also occur in this situation).
In most situations, the dominating source of "comfort" for a colonist is sleeping in a bed. Even a simple sleeping spot provides 0.4 of comfort and would leave the pawn at 40% of comfort need after sleeping on it for a few hours.
To obtain comfort need gains, the pawn has to be actually "using" the piece of furniture, such as doing work on a workbench with a chair, or actively resting in a bed. It is not sufficient to be merely standing or lying on top of the furniture.
Furniture stat
Every piece of furniture has a comfort stat, listed in the item's information panel. The comfort stat is determined by the base value for the item, the item's quality level, and sometimes other nearby pieces of furniture providing an "offset" to the base value.
Example: a bed of "normal" quality has a base value of 0.75, that can be modified with an adjacent end table by +0.05, as well as a nearby dresser by another +0.05, yielding an effective comfort stat for this bed of 0.85.
Furniture of normal quality has the following comfort stat values:
(Animal beds and sleeping boxes have no effect, despite having a comfort stat, because animals do not have a comfort need that could be satisfied. Currently the animal furniture merely marks a spot for animals to sleep or rest on, unless mods are used that enhance this behavior.)
Strategy
Since comfort levels between 10% and 60% are mood-neutral, furniture with less than 0.6 comfort is almost ineffective with regards to mood. This is because even a sleeping spot would leave the pawn at 40% comfort when beginning a workday, and it would then require about 6 hours of complete comfort starvation to let the level drop below 10%. Situations like this rarely happen in actual games, and if they do, the -3 mood penalty from the "Uncomfortable" thought is likely relatively meaningless.
In practice, this means that especially stools in front of workbenches are almost useless, because all they do is keep the comfort level at 50%. However, a pawn who slept in a bed even of poor quality runs a rather low risk of dropping to 10% comfort level regardless if stools are used or not.
In other cases, the comfort stat of furniture is impactful only if the furniture is used for non-trivial durations – this is mainly the case for all kinds of beds as well as work station chairs that are used intensively.
Interestingly, putting highly comfortable furniture into the recreation area is very often not worth it either (apart from the high beauty rating of these items). This is because "recreation furniture" is, even in the best case, not used for longer than two hours, and is usually followed by a longer sleeping period anyways. High comfort stats on these items therefore do not matter in practice.
It is much better to have a "masterwork armchair" in front of a research desk than in front of a TV in the rec room (even though the latter might be more intuitive).