You may not need to worry about whether the playback device will be a firestick if it regular video subtitles you are looking for. Video subtitles are not generally associated with a particular device, unless you are intending to make them on the fly on that particular device - see note below.
They are usually generated on the 'server side' and stored or transmitted with the video. They can be contained in a separate subtitle track in the video container, which gives the flexibility to turn them on or off at playback and may also allow you support multiple language versions. Alternatively they can be 'burnt' into the video itself.
It is possible to generate the subtitles automatically or manually, or to combine the two - i.e. generate automatically and then clean up manually.
Automatic generation is still evolving, much of it AI based, and is not perfect - look at some examples on YouTube and you can see that even with their resources not all words are captured correctly. You can experiment with free versions of tools like Media.io (https://www.media.io/online-auto-subtitle-generator.html), Veed (https://www.veed.io/tools/auto-subtitle-generator-online) and others.
Note that there are technologies to generate captions on the device side also - Chrome has this capability built in to the browser itself as an option under the Accessibility settings. This works independently of the video, or audio, being played and will work in real time:
