That person captured the cub and brought it to the RSPCA's Newbrook Farm Animal Hospital in Birmingham.
"This poor little fox cub had got herself in a real pickle," veterinary nurse Samantha Belding said.
"She had managed to force her head into the old plastic jar, but then couldn't get it out, as her ears were trapping her. We think she had been stuck like this for quite a few days, as flies had already laid eggs on a wound that had formed on her neck where the jar was rubbing her," Belding said.
The fox was cut free from the jar and hospital staff treated her wound with antibiotics and pain relievers.
"Thankfully, it turned out to be a superficial wound, so she only needed to stay at our hospital for a day before she was transferred to a specialist wildlife center," Belding said.
The fox is expected to be released back into the wild.
The RSPCA said the incident should serve as a reminder to the public of the dangers litter can pose to wildlife.
