The MPL-2.0 license does not require that Contributors add a copyright notice or in another way mark files as changed and it also doesn't contain any explicit clauses about mis-representation.
Based on that, there seems to be no violation of the license, although you might look deeper in the copyright law of your country if it states anything about mis-representation by the appearance of attributing modifications to you. Thus there is nothing you can do about it.
In any case, the fact that this Contributor did not add a copyright notice does not mean the copyright now belongs to you. The copyright notice is just a means to make it explicit who owns the copyrights, but this Contributor owns the copyrights on their changes regardless.
As clause 3.4 quite explicitly states that copyright notices may not be altered, there is a possibility that this Contributor (mistakenly) believes they are not allowed to add their own copyright notice. In that case, a friendly note asking them to add their copyright claim should be enough to set this straight.