In Firefox, in the downloads window (Ctrl+J), there's a little "Play" arrow for downloads which have been stopped; that should make it resume.
But for big files, I would generally not trust a browser's built-in features; instead, I'd use a purpose-built downloader, which comes with support for resuming if it's any use at all. (I personally like GetRight.)
Another handy feature is download acceleration by downloading the file using several connections at once: e.g. for three streams, it'll download the first third of the file using one connection, the second third using another one and the third third using another one, all in parallel. Useful for spreading the load across multiple servers, for things that are mirrored; but sometimes even with multiple connections to a single server if it caps upload bandwidth per connection.
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But for big files, I would generally not trust a browser's built-in features; instead, I'd use a purpose-built downloader, which comes with support for resuming if it's any use at all. (I personally like GetRight.)
Another handy feature is download acceleration by downloading the file using several connections at once: e.g. for three streams, it'll download the first third of the file using one connection, the second third using another one and the third third using another one, all in parallel. Useful for spreading the load across multiple servers, for things that are mirrored; but sometimes even with multiple connections to a single server if it caps upload bandwidth per connection.