if you are of the opinion that you can do the job then I see no need to tell the employer - if you think you can't do the job then you should not have applied.
My opinion:
Of course, it is a person's choice whether or not they disclose a disability or other condition to the prospective employer.
Fibromyalgia may not be classified as a disability in most places. However, as Hope said above,
it leads to or causes disability at least some of the time in most people who have it. If a person cannot drive, or cannot walk, or whatever, that is a disability and it would keep them from going to work that day or days. In my opinion, having a chronic pain condition is in itself a kind of disability. there are many kinds of disabilities, and not all of them mean that you cannot
ever do what you want to do, just as not all of them are visible.
As for being able to do the job or not:
If what you mean by "able to do the job" is being 100% on every day and equally able to to the job every day, then no one with fibromyalgia is "able to do" any job, because there will always be times we don't function as well due to fibro. A person is fully able to do a job on the days that the flare is not so bad they cannot make it to work, and there's no reason they shouldn't apply for and get the job.
If, however, they don't disclose the fact that there will inevitably be days when they are slower at work than normal, or cannot come to work at all, they are being deceitful.
If you miss one day a month of work or have an off day now and then no one will really notice. But if it is a regular thing, as it is very likely to be with someone who has fibromyalgia (unless it is a very mild form that doesn't impede the person's activities at all), the employer and fellow workers will notive and wonder. And then it will come out that this is chronic, won't go away, and was known to the employee before they were hired.
As has been said before, this will break trust with the employer....not a good idea.
If a person has a disability that
won't ever affect their job performance (such as having no sense of smell, but your job doesn't involve having to smell anything), then and only then it is OK not to mention it.
Of course medical conditions are none of the employer's business.....unless they are going to affect the job performance. And not only personal disability should be reported to the employer if it will affect being on the job, but other things as well. For instance, if you have a child with a disability or an illness that may mean you have to leave work on occasion to take the child to the hospital, you should disclose that as well.
Being upfront is always best with employers. Many are sympathetic, want to help, and are willing to make accommodations when the employee is straightforward with them, but won't like it at all if they feel something that affects the person's job performance has been hidden from them.
And even if in a particular place a person cannot be actually fired for having a disability, damaging the relationship between employee and employer is never good.