I understand that Firefox is blocking files that are being download, please first try:

• What to do if you can't download or save files

Does it say:

Blocked: May contain a virus or spyware — example.com

That message most likely is generated by the Safe Browsing filter, which is a service based on site reputation data from Google. Something changed recently and this service is blocking many more sites.

Unlike the download block notice in Google Chrome, Firefox does not currently have a button to override the block. That is under consideration for a future version.

Currently, the only workaround for this is to temporarily turn off Safe Browsing, do the download, then turn it back on again. Or since you are using a Trend Micro product, you might already have protection against accessing untrustworthy websites in which case you could just turn it off. (Please check your product information.)

The setting is here:

"3-bar" menu button (or Tools menu) > Options > Security panel

It's the checkbox for "Block reported attack sites"

I have attached a screen shot in case it's helpful.