The fundamental question to ask is whether you have modified MPL-licensed source code. It can be generally checked easily because files under the MPL license are signaled with the typical header:
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
If a file that you have modified is not MPL-licensed, you have first to clarify under which alternative license it was made available to you. It could be under a more permissive license such as MIT. In this case, you would not be obligated to share the modifications, provided that you respect the conditions of the alternative license.
Most importantly, you do not have to share any source file that you entirely created yourself. From here:
new files containing no MPL-licensed code are not Modifications, and therefore do not need to be distributed under the terms of the MPL
If you have modified MPL-licensed files, then you must share the modifications. Quoting from the MPL FAQ:
Q9: I want to distribute (outside my organization) MPL-licensed source code that I have modified. What do I have to do?
To see the complete set of requirements, read the license. However, generally:
You must inform the recipients that the source code is made available to them under the terms of the MPL (Section 3.1), including any Modifications (as defined in Section 1.10) that you have created.
You must make the grants described in Section 2 of the license.
You must respect the restrictions on removing or altering notices in the source code (Section 3.4).
Q10: I want to distribute (outside my organization) an executable program based on MPL-licensed source code that I have modified. What do I have to do?
You must make available the MPL-licensed portions of the source code as described in the previous question, and inform the recipients how they can obtain such source code (Section 3.2).
And finally, yes providing a notice with a link to the source code is sufficient. Whether it is the original source code or the source code of your modified version depends on the question above.