Firstly, "first-line manager/supervisor" and "first-line employee/worker" are not the same thing. The former refers to the lowest tier of management: those who manage non-managers. But applied to employees or workers, "first-line" means those (as you might say) "at the sharp end". The term is really quite similar in meaning to "front-line".

When it is a company or organisation dealing with the public in some way (particular in service industries), "first-line" workers are those who actually deal directly with customers or other members of the public. However, that is not relevant in a manufacturing company like this one. Here they are employees engaged directly in the manufacturing process, as opposed to those doing ancillary tasks (clerical, packing, etc.).

"As of Oct. 1, the monthly wage for all "first line" workers—those on the assembly lines—as well as their line leaders and supervisors, will be increased to 2,000 yuan, or about $293, Hon Hai said in a statement."
(Wall Street Journal: a reliable native source.)
http://online.wsj.com/news/articles/SB1000142405274870472610...

"The traditional organizational structure was dismantled. First-line employees were organized into self-directed work teams. These employees, known as "members", were given a wide array of responsibilities [...] Middle managers, formerly responsible for the tasks turned over to the first-line workers, now focused their efforts on teaching and on coaching members to lead themselves. [...] Slowly, members working on the production line began to take over more of the responsibility for operating Johnsonville Foods [...]"
http://books.google.es/books?id=HQ6iDuVQPEIC&pg=PT77&lpg=PT7...

"Unfortunately, managers and executives are all too often unwilling to hear what the first-line workers have to say—or listen to what they have to say with a jaundiced ear, thinking that the workers’ only interest is to make their own tasks easier or less stressful."
http://www.rklesolutions.com/blog/making-strategy-effective/

"First line workers responsible for operating tasks (semi-skilled)"
http://upetd.up.ac.za/thesis/available/etd-03162005-151333/u... (foot p. 16)

" Of these workers, men's jobs were skilled or semiskilled: drivers and mechanics, irrigation work, handhoeing, melon picking, and pepper picking. Women's jobs were melon and pepper packing; only during the bean harvest were women first-line workers."
http://www.popline.org/node/378923