Two circles $\mathcal{C}_1$ and $\mathcal{C}_2$ with centers $(1, 1)$ and $(4, 5)$ and radii $r_1 < r_2$, respectively, are drawn on the coordinate plane. The product of the slopes of the two common external tangents of $\mathcal{C}_1$ and $\mathcal{C}_2$ is $3$. If the value of $(r_2 - r_1)^2$ can be expressed as a common fraction in the form $\tfrac{m}{n}$ where $m$ and $n$ are relatively prime positive integers, find $m + n$. 
1. Let the centers of the circles $\mathcal{C}_1$ and $\mathcal{C}_2$ be $O_1$ and $O_2$ respectively, with coordinates $(1, 1)$ and $(4, 5)$. The radii of the circles are $r_1$ and $r_2$ respectively, with $r_1 < r_2$.
2. The distance between the centers $O_1$ and $O_2$ is calculated as:
   \[
   d = \sqrt{(4 - 1)^2 + (5 - 1)^2} = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5
   \]
3. Let $P$ be the intersection point of the common external tangents, and let $X$ and $Y$ be the points of tangency on $\mathcal{C}_1$ and $\mathcal{C}_2$ respectively.
4. The product of the slopes of the two common external tangents is given as $3$. Let $\theta$ be the angle $\angle YPO_2$ and $\alpha$ be the acute angle that $\overline{PO_2}$ makes with the horizontal.
5. The product of the slopes of the tangents can be expressed as:
   \[
   \frac{\tan\alpha + \tan\theta}{1 - \tan\alpha \tan\theta} \cdot \frac{\tan\alpha - \tan\theta}{1 + \tan\alpha \tan\theta} = 3
   \]
6. Given that the slope of $\overline{O_1O_2}$ is $\frac{4}{3}$, we have:
   \[
   \tan\alpha = \frac{4}{3}
   \]
7. Substituting $\tan\alpha = \frac{4}{3}$ into the product of slopes equation:
   \[
   \frac{\frac{4}{3} + \tan\theta}{1 - \frac{4}{3} \tan\theta} \cdot \frac{\frac{4}{3} - \tan\theta}{1 + \frac{4}{3} \tan\theta} = 3
   \]
8. Simplifying the above expression:
   \[
   \frac{\frac{4}{3} + \tan\theta}{1 - \frac{4}{3} \tan\theta} \cdot \frac{\frac{4}{3} - \tan\theta}{1 + \frac{4}{3} \tan\theta} = \frac{\frac{16}{9} - \tan^2\theta}{1 - \frac{16}{9} \tan^2\theta} = 3
   \]
9. Let $u = \tan^2\theta$. Then:
   \[
   \frac{\frac{16}{9} - u}{1 - \frac{16}{9} u} = 3
   \]
10. Solving for $u$:
    \[
    \frac{16 - 9u}{9 - 16u} = 3 \implies 16 - 9u = 27 - 48u \implies 39u = 11 \implies u = \frac{11}{39}
    \]
11. Since $\tan^2\theta = \frac{11}{39}$, we have:
    \[
    \sin^2\theta = \frac{11}{50}
    \]
12. Using the similarity of triangles $\triangle PO_2Y \sim \triangle PO_1X$, we have:
    \[
    \frac{5}{r_2 - r_1} = \sin\theta
    \]
13. Therefore:
    \[
    \sin^2\theta = \left(\frac{5}{r_2 - r_1}\right)^2 = \frac{11}{50}
    \]
14. Solving for $(r_2 - r_1)^2$:
    \[
    (r_2 - r_1)^2 = 25 \sin^2\theta = 25 \cdot \frac{11}{50} = \frac{11}{2}
    \]
15. The fraction $\frac{11}{2}$ is in simplest form, so $m = 11$ and $n = 2$. Therefore, $m + n = 13$.

The final answer is $\boxed{13}$.