I could not find any evidence that this ever happened as you described it, and Snopes concurs. In fact, mass employee layoffs without advanced warning is (under almost all circumstances) illegal in the United States. "Advanced warning" just means that the entire workforce needs to be warned that a mass layoff is going to happen at least 60 days before the first employee is notified that they specifically have been chosen to be terminated. So, any company that pulled such a stunt would almost certainly be breaking the law, and would likely have received a lot of negative press for it. Since I could find no such record in the press, I think it's relatively safe to say that it is just a legend, at least within the United States. Proving whether or not it happened internationally may be impossible unless there is a high profile incident that somehow eluded our collective Google-fu.
I will say that for large layoffs that are below the legal definition of a "mass layoff," using this technique would be legal in the US, and therefore might be plausible. However, I would think that such a stunt would cause a lot of negative press for the employer. The same sort of effect could be achieved by simply packing up all of the terminated employees' belongings over a weekend and then individually denying them access to the facility on Monday morning, which avoids the spectacle of having everyone laid off in a group.
There have been a couple reported incidents both in the UK and in the USA where the entire workforce was terminated using the "fire drill" method. In all such cases, however, it was due to the entire company ceasing business, so all employees were terminated.
Addition (2014-02-21): It turns out that there has been an international treaty (currently ratified by 35 countries) in effect since 1985, called the Termination of Employment Convention, that has very similar restrictions to the US law I mentioned above.
Article 11: A worker whose employment is to be terminated shall be entitled to a reasonable period of notice or compensation in lieu thereof…
As was mentioned in the comments, the employer could always immediately revoke access to the employee(s) and then provide some severance compensation, but unless the employees were being terminated for negligence (which sounds not to be the case in the supposed firedrill scenario), then the employer would likely want to have the employees work for that period anyway.
Article 13 Paragraph 1: When the employer contemplates terminations for reasons of an economic, technological, structural or similar nature, the employer shall: (a) provide the workers' representatives concerned in good time with relevant information including the reasons for the terminations contemplated, the number and categories of workers likely to be affected and the period over which the terminations are intended to be carried out; (b) give, in accordance with national law and practice, the workers' representatives concerned, as early as possible, an opportunity for consultation on measures to be taken to avert or to minimise the terminations and measures to mitigate the adverse effects of any terminations on the workers concerned such as finding alternative employment.
So, in all 35 countries that ratified that treaty, the employer would have had to give the employees advanced notice of the layoffs.
